Cavity ring-down spectroscopy system and method for modulating light beam therein

Abstract

A cavity ring-down spectroscopy system and a method for modulating a light beam therein are provided. The cavity ring-down spectroscopy system comprises: at least one laser generating a light beam; a first optical modulator positioned to attenuate the light beam from the at least one laser; a second optical modulator positioned to attenuate the light beam from the first optical modulator; a ring-down cavity positioned to receive the light beam from the second optical modulator; and at least one light sensor detecting the intensity of light leaked from the ring-down cavity.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 62 / 828,750, filed on April 3, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates generally to gaseous sample analysis and, more particularly, to a cavity ring-down spectroscopy system and a method for modulating an optical beam therein. Background Art

[0004] Cavity ring-down spectroscopy ("CRDS") is a method commonly used to identify and quantify individual analytes in gaseous samples using their absorption spectra. A typical CRDS system employs a laser that generates a light beam that is directed into a cavity of a chamber having two highly reflective mirrors. The light beam is typically within the visible spectrum, often within the near infrared ("IR") spectrum, and is tuned to a single wavelength to identify the presence of individual molecules. The light beam is then repeatedly reflected between the mirrors, which allows a portion of the light to escape the ring-down cavity. When the laser resonates with the cavity mode, intensity is generated in the cavity due to constructive interference. When the light entering the cavity is eliminated, the intensity of the light in the ring-down cavity decays at a predetermined rate when it is empty. A small portion of the light escapes from the ring-down cavity without being reflected by the mirrors. The intensity of the escaping light is measured by a sensor component to determine the decay rate.

[0005] When a gaseous sample is placed in a ring-down cavity, the analytes present in the gaseous sample absorb some of the light, thereby accelerating the decay of the light intensity in the ring-down cavity. An absorption spectrum is generated by measuring the decay time of light at specific wavelengths when a gaseous sample is present relative to the decay time of light at these wavelengths when no gaseous sample is present. Linear regression of the measured absorption spectrum of the gaseous sample with the known absorption spectra of various analytes enables identification and quantification of individual analytes in the gaseous sample. Summary of the invention

[0006] In one aspect, a cavity ring-down spectroscopy system is provided, comprising: at least one laser that generates a light beam; a first optical modulator positioned to attenuate the light beam from the at least one laser; a second optical modulator positioned to attenuate the light beam from the first optical modulator; a ring-down cavity positioned to receive the light beam from the second optical modulator; and at least one light sensor that measures the intensity of light leaking from the ring-down cavity.

[0007] The cavity ring-down spectroscopy system may further include at least one focusing lens for focusing the light beam to match an optical mode of the ring-down cavity.

[0008] The first light modulator and the second light modulator may be an acousto-optic modulator.

[0009] The cavity ring-down spectroscopy system may further include a controller configured to guide the first optical modulator to attenuate the light beam at or close to the attenuation limit of the first optical modulator, and simultaneously guide the second optical modulator to further attenuate the light beam at or close to the attenuation limit of the second optical modulator to reduce or eliminate the intensity of the light beam from the first optical modulator.

[0010] On the other hand, a method for modulating a light beam in a cavity ring-down spectroscopy system is provided, the method comprising: generating a light beam via at least one laser; eliminating the light beam from the at least one laser via a first optical modulator; eliminating the light beam from the first optical modulator via a second optical modulator; receiving the light beam attenuated by the second optical modulator in a ring-down cavity; and measuring the decay rate of light leaking from the ring-down cavity.

[0011] The first light modulator and the second light modulator may be an acousto-optic modulator.

[0012] The method may further include: guiding the first optical modulator to attenuate the light beam at or near the attenuation limit of the first optical modulator; and simultaneously guiding the second optical modulator to further attenuate the light beam traveling toward the ring-down cavity at or near the attenuation limit of the second optical modulator to reduce or eliminate the intensity of the light beam from the first optical modulator.

[0013] Other technical advantages will become apparent to one of ordinary skill in the art after reviewing the following drawings and description. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a better understanding of one or more embodiments described herein and to more clearly show how the same may be implemented, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0015] Figure 1 is a schematic diagram of various optical and pneumatic components of a cavity ring-down spectroscopy system according to one embodiment; and

[0016] Figure 2 It is used to control Figure 1 The schematic diagram of the electrical control system of the various optical and pneumatic components of the cavity ring-down system is shown.

[0017] Unless otherwise specifically noted, the articles shown in the drawings are not necessarily drawn to scale. DETAILED DESCRIPTION

[0018] For simplicity and clarity of explanation, reference numerals in the drawings may be repeated, where deemed appropriate, to indicate or similar elements. In addition, in order to provide a thorough understanding of one or more embodiments or multiple embodiments described herein. However, those skilled in the art will appreciate that the embodiments described herein may be practiced without these specific details. In other cases, well-known methods, processes, and components are not described in detail to avoid confusing the embodiments described herein. First, it should be understood that although exemplary embodiments are shown in the drawings and described below, any number of technologies may be used to implement the principles of the present disclosure, whether currently known or not. The present disclosure should in no way be limited to the exemplary implementations and technologies shown in the drawings and described below.

[0019] The various terms used throughout this specification may be read and understood as follows, unless the context dictates otherwise: "or" as used throughout is inclusive, as if written as "and / or"; singular articles and pronouns as used throughout include their plural forms and vice versa; similarly, gendered pronouns include their corresponding pronouns, so that the pronouns should not be construed as limiting anything described herein to use, implementation, performance, etc. by a single gender; "exemplary" should be understood as "illustrative" or "exemplary", and not necessarily as "preferred" over other embodiments. Further definitions of terms may be set forth herein; these may apply to prior and subsequent instances of these terms, as will be understood by reading this specification.

[0020] Without departing from the scope of the present disclosure, the systems, devices and methods described herein may be modified, added to or omitted. For example, the components of the systems and devices may be integrated or separated. In addition, the operations of the systems and devices disclosed herein may be performed by more, fewer or other components, and the methods described may include more, fewer or other steps. In addition, 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.

[0021] Any module, unit, component, server, computer, terminal, engine or device illustrated herein that executes instructions may include or otherwise access computer-readable media, such as storage media, computer storage media or data storage devices (removable and / or non-removable), such as, for example, disks, optical disks or tapes. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented by any method or technology for storing information such as computer-readable instructions, data structures, program modules or other data. Examples of computer storage media include RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disk (DVD) or other optical storage, cassettes, tapes, disk storage or other magnetic storage devices, or any other medium that can be used to store required information and can be accessed by applications, modules or both. Any such computer storage medium may be part of a device or accessible or connectable thereto. In addition, unless the context clearly indicates otherwise, any processor or controller set forth herein may be implemented as a single processor or multiple processors. Multiple processors may be arrayed or distributed, and any processing function mentioned herein may be performed by one or more processors, even if a single processor may be illustrated. Any methods, applications, or modules described herein may be implemented using computer-readable / executable instructions, which may be stored or otherwise held by such computer-readable media and executed by one or more processors.

[0022] CRDS systems typically include an optical modulator for modulating light from a laser. The optical modulator can deflect light to another path in order to reduce the intensity or power of the light. An acousto-optic modulator (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, generating sound waves in the material. These sound waves expand and compress the material, thereby generating periodic changes in the refractive index and allowing Bragg diffraction. Light entering the AOM at a first-order Bragg angle relative to a plane perpendicular to the axis of propagation of the sound wave will be deflected at maximum efficiency by an amount equal to twice the Bragg angle. Eliminating the electrical signal eliminates the Bragg diffraction properties of the material and allows the light to pass through without deflection, effectively attenuating the light along the deflected optical path. A byproduct of the AOM is that the frequency of the deflected light is shifted.

[0023] An electro-optic modulator is another type of light modulator that applies a DC or low-frequency electric field to a material to distort the position, orientation, and / or shape of the material's molecules. As a result, the refractive index is changed to change the phase of the outgoing beam as a function of the applied field. Phase modulation is converted into intensity modulation by sending the beam through a polarizer. In another approach, a phase modulator can act as an intensity modulator when placed in the branches of an interferometer.

[0024] Optical modulators are used to control the intensity of a beam of light generated by a laser. One side effect of using an AOM is that the frequency of the light is shifted. This shift is small relative to the absolute frequency of the light.

[0025] In CRDS, the decay rate of light in a ring-down cavity is determined in order to understand the absorption spectrum of a gaseous sample in the ring-down cavity. However, conventional CRDS systems that employ a single optical modulator have difficulty eliminating the light provided to the ring-down cavity as quickly and completely as desired. This is more true for the mid-IR than for the near-IR and visible wavelengths. Typically, optical modulators do not generate a perfect step function output and have ramp-up and ramp-down times. As a result, the additional light that enters the ring-down cavity at the beginning of a ring-down event can be difficult to compensate for, making linear regression of a known analyte absorption spectrum more challenging.

[0026] Figure 1 2 shows various components of a CRDS system 20 according to a particular embodiment. A CO2 laser 24 and a carbon 13O2 laser 28 are provided. The CO2 laser 24 and the carbon 13O2 laser 28 are gas tube lasers that emit at a series of quasi-uniformly spaced known frequencies that can be quickly selected using an adjustable diffraction grating arrangement. Gas tube laser technology has a long history and is a stable and robust way to generate infrared radiation at precisely known frequencies. Both the CO2 laser 24 and the carbon 13O2 laser 28 emit light in the mid-IR spectrum.

[0027] Each of the CO2 laser 24 and the Carbon 13O2 laser 28 has an actuator and output coupler that is capable of adjusting the length of the laser cavity and changing the angle of the grating at the back of the cavity, thereby changing its pitch to adjust the wavelength it reflects. By adjusting the length of the laser cavity and changing the angle of the grating, the laser can be very accurately tuned to a specific wavelength and desired mode quality.

[0028] The CO2 laser 24 generates a first laser beam 32, and the carbon 13O2 laser 28 generates a second laser beam 36. Depending on the desired optical frequency, the CO2 laser 24 is either tuned and the first laser beam 32 is generated while the carbon 13O2 laser 28 is detuned, or the carbon 13O2 laser 28 is tuned and the second laser beam 36 is generated while the CO2 laser 24 is detuned. In this way, at most only one of the CO2 laser 24 and the carbon 13O2 laser 28 outputs a beam at any particular time, so that the first beam 32 and the second beam 36 are not combined at the same time. Mid-infrared, particularly long-wavelength infrared, is selected as the type of light because most volatile organic compounds absorb light in this range. As a result, a variety of volatile organic compounds can be measured by a single system. The CO2 laser operates in this range and has sufficient power and line width narrowness for ring-down spectroscopy. The use of two lasers increases the range and number of available wavelengths that the CRDS system 20 can use to analyze gaseous samples.

[0029] The first laser beam 32 is redirected via a mirror 40 on the optical mount to a beam splitter 44. The beam splitter 44 is partially reflective and partially transmissive and splits each of the first laser beam 32 and the second laser beam 36 into two beams, a sampling beam 48 and a working beam 52 having the same characteristics as the sampling beam 48 and may have a similar intensity as the sampling beam 48.

[0030] The sampling beam 48 is received by a fast detector 56. The fast 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 caused by suboptimal tuning of the CO2 laser 24 or the carbon 13O2 laser 28. In response to detecting an undesirable beat frequency, the corresponding laser 24 or 28 is tuned 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 tuned to a different wavelength.

[0031] The working beam 52 continues to the first optical modulator 60, which then deflects the working beam 52 to a reflector 64 on an optical mount. The reflector 64 redirects the light to the second optical modulator 68, which in turn deflects the working beam 52 to a focusing lens 72. In this embodiment, the first optical modulator 60 and the second optical modulator 68 are AOMs, also known as Bragg devices, but may also be electro-optic modulators in other embodiments.

[0032] The first optical modulator 60 and the second optical modulator 68 act as attenuators to adjust the intensity of the working beam 52 and eliminate the beam when a ring-down event begins. Because they are AOMs, the first optical modulator 60 and the second optical modulator 68 use the acousto-optic effect to diffract light using sound waves (usually at radio frequencies). In each of the first optical modulator and the second optical modulator, a piezoelectric transducer is coupled to a material such as germanium or glass, and an oscillating electrical signal is used to oscillate the piezoelectric transducer. The vibrating piezoelectric transducer generates sound waves in the material, which expand and compress the material, thereby generating periodic changes in the refractive index and allowing Bragg diffraction. Light entering the AOM at a Bragg angle relative to a plane perpendicular to the axis of propagation of the sound wave will be deflected at maximum efficiency by an amount equal to twice the Bragg angle. Eliminating the electrical signal eliminates the Bragg diffraction properties of the material and allows the light to pass through without deflection, effectively eliminating the light along the deflected light path. Therefore, the intensity of the sound can be used to modulate the intensity of the light in the deflected beam.

[0033] 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%, which represents 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 is about 0.1% of the input light intensity. When the acoustic wave applied to the germanium is turned off, the deflected beam loses about 30dB or 99.9% of the previous intensity. The attenuation limit means the upper limit of how much the optical modulator can reduce the input light intensity.

[0034] Optical modulators are asymmetric because, as a side effect, when receiving input light at their first end, they Doppler shift the frequency of the light in a first mode, and when receiving input light at their second end, they Doppler shift the frequency of the light in a second mode that is opposite to the first mode and attenuates the same power. However, the Doppler shift of the frequency of the light is in the same direction regardless of whether the light enters at the first or second end.

[0035] Conventional CRDS systems use a single optical modulator and therefore have a frequency-shifted operating beam. These frequency shifts are typically small relative to the frequency of the light and can change the way the light is absorbed by the material in the cavity, but this frequency shift can be compensated for during analysis. If the diffraction is toward the source of the acoustic wave in the AOM, the frequency shift is downward, while if the diffraction is away from the source of the acoustic wave, the frequency shift is upward. As discussed, the effect is minimal.

[0036] The working beam 52 deflected by the second optical modulator 68 is focused via the focusing lens 72 to match the optical mode of the ring-down cavity 84. The laser beam, and therefore the working beam 52, continues to diverge as it travels from the CO2 laser 24 or the carbon 13O2 laser 28. The focusing lens 72 focuses the working beam 52 downward.

[0037] A mirror 76 on the optical mount then redirects the working beam 52 to the ring-down chamber 80. The two mirrors 64, 76 extend the path length of the working beam 52.

[0038] The ring-down chamber 80 is an elongated tube in which a ring-down cavity 84 is defined. A front cavity mirror 88a and a rear cavity mirror 88b (also alternatively referred to herein as cavity mirrors 88) are located at the longitudinal ends of the ring-down cavity 84. The cavity mirrors 88 are highly reflective, both for light directed to the cavity mirrors 88 from outside the ring-down cavity 84 and for light directed to the cavity mirrors 88 within the ring-down cavity 84. As a result, a portion (about 0.1%) of the working light beam 52 is directed to the front cavity mirror 88a, passes through the front cavity mirror 88a, and enters the ring-down cavity 84, while a majority (about 99.9%) of the working light beam 52 is reflected back to the mirror 76.

[0039] The cavity mirror 88 is mounted on a mirror mount 92, which can be actuated to adjust the position and orientation of the cavity mirror 88. In particular, the front cavity mirror 88a facing the front of the ring-down cavity 84 is mounted on the mirror mount 92, which can be actuated via three motorized micrometers 96a. The rear cavity mirror 88b facing the rear of the ring-down cavity 84 is mounted on the mirror mount 92, which can be actuated via three motorized micrometers 96b, which can be manually adjusted for optical alignment, or actuated by a piezoelectric device, which allows them to be further adjusted by a piezoelectric driver.

[0040] The angle of each cavity mirror 88 can be changed so that they are perfectly aligned so that the light beam does not deviate when it enters the ring-down cavity 84. If one of the cavity mirrors 88 is askew, some of the light is reflected to the side of the ring-down cavity 84, losing the intensity of the light and generating higher-order modes. The micrometer 96 can also be adjusted simultaneously to change the length of the ring-down cavity 84. This allows the ring-down cavity 84 to be tuned so that the ring-down cavity 84 resonates at the frequency of the light entering the ring-down cavity 84.

[0041] The focusing lens 72 focuses the laser light to match the optical mode of the ring-down cavity 84 , so that the minimum beam waist of the light beam is located at the same position as the minimum beam waist of the ring-down cavity 84 .

[0042] A light sensor in the form of a liquid nitrogen cooled detector 100 is located behind the rear cavity mirror 88b to receive light escaping therethrough. The liquid nitrogen cooled detector 100 measures the intensity of light escaping from the ring down cavity 84. Other types of sensors for measuring the intensity of escaping light may be used in place of the liquid nitrogen cooled detector 100, such as a thermoelectric cooled detector.

[0043] During the tuning process, one of the CO2 laser 24 and the carbon 13O2 laser 28 is tuned and the sample beam 48 arriving at the fast detector 56 is analyzed to identify the beat frequency. If a beat frequency is present in the sample beam 48, the corresponding laser is adjusted until the undesirable beat frequency is removed or the amplitude is reduced to below acceptable limits.

[0044] In addition, the position of cavity mirror 88 is adjusted via micrometer 96 so that no high-order modes exist in ring-down cavity 84 .

[0045] The gaseous sample is loaded into the ring-down chamber 84 from a thermal desorption tube 104 used to collect the gaseous sample for testing. The thermal desorption tube is typically made of stainless steel and contains various types of solid adsorbent materials. The solid adsorbent is selected for sampling specific compounds to capture and retain the compound of interest, even in the presence of other compounds, and allows the collected compound to be easily desorbed or extracted for analysis. In addition, the selected solid adsorbent does not react with the compound of interest.

[0046] In one example, the gaseous sample is a human breath sample collected from a patient. The receiving end 108 of the thermal desorption tube 104 receives the human breath collected from the human body for testing. As a result, the compounds of interest are more concentrated at the receiving end 108 of the thermal desorption tube 104.

[0047] The pneumatic system 112 is used to load the gaseous sample from the thermal desorption tube 104 into the ring-down chamber 84 and to evacuate the pneumatic system 112 containing the ring-down chamber 84. During the loading of the gaseous sample, the pneumatic system 112 fills the ring-down chamber 84 with the gaseous sample that has been collected (i.e., desorbs the gaseous sample from the thermal desorption tube 104 and places the gaseous sample into the ring-down chamber 84 without introducing contaminants), brings the pressure and temperature in the ring-down chamber to one atmosphere and 50 degrees Celsius, and seals the ring-down chamber 84. In this embodiment, the absorption spectra of a set of samples to which the measured absorption spectra are compared are determined at this pressure and temperature to ensure consistency between these parameters, which can affect the results. However, in other embodiments, the pressure and temperature can be fixed at other levels for known and measured absorption spectra. During the evacuation of the gaseous sample, the pneumatic system 112 cleans the previously provided gaseous sample from the ring-down chamber 84 and the various conduits used to guide the gaseous sample from the thermal desorption tube 104 to the ring-down chamber 84.

[0048] The pneumatic system 112 has an inlet portion that includes a nitrogen source 116. The nitrogen source 116 is a very clean nitrogen supply that is pressurized or can pressurize nitrogen to at least one atmosphere above. In the present embodiment, the nitrogen source 116 is pressurized at a pressure of 5 psi above ambient pressure, but can be changed as long as the pressure is sufficient to pressurize the ring-down chamber 84 to one atmosphere, or some other selected atmosphere for analysis. In the illustrated embodiment, the nitrogen source 116 is nitrogen that evaporates from a liquid nitrogen reservoir. The nitrogen source 116 is connected to a gas inlet valve 124a via a conduit 120. An auxiliary gas inlet valve 124b can be connected to other gases, but is not often used. The gas inlet and auxiliary gas inlet valves 124a, 124b are in communication with the inlet line 120a. A pressure gauge 128 is positioned along the inlet line 120a and the inlet line valve 124c. A filter 130a is positioned along the inlet line 120a before the cavity inlet valve 124d, which seals the inlet line 120a from the ring-down cavity 84. The filter 130a blocks contaminants from entering the ring-down cavity 84, where they can deposit on the cavity mirror 88 and interfere with reflections.

[0049] The gas inlet valve 124a and the auxiliary gas inlet valve 124b are in communication with the access valve 124e. The access valve 124e allows or prohibits direct access to the desorption tube pipeline 120b and the sample outlet pipeline 120c.

[0050] The desorption line 120b includes a front valve 124f and a rear valve 124g. The thermal desorption tube 104 is located between the front valve 124f and the rear valve 124g, and the receiving end 108 of the thermal desorption tube 104 is located toward the rear valve 124g. The thermal desorption tube 104 is located in the heater 132.

[0051] The sample outlet line 120 c includes a sample outlet valve 124 h and a mass flow controller 136 .

[0052] The pneumatic system 112 also has an outlet portion that includes a chamber outlet valve 124i that communicates with the ring-down chamber 84. An outlet line 140 communicates with the chamber outlet valve 124i. A pressure gauge 144 is located along the outlet line 140. A vacuum shutoff valve 124j is located between the pressure gauge 144 and a vacuum pump 148. A vacuum suction valve 124k communicates with the vacuum pump 148 and draws air through a pump suction line 150. A filter 130b is located in the pump suction line 150 to prevent contaminants that may interfere with the operation of the vacuum pump 148 from entering.

[0053] Valves 124a - 124k may also be alternatively referred to herein as valves 124 .

[0054] Although the chamber inlet valve 124d and the chamber outlet valve 124i are shown coupled to the ring-down chamber 84 at certain locations for convenience, it should be understood that the locations at which the valves 124d, 124i are coupled to the ring-down chamber 84 may vary. In a preferred configuration, the chamber inlet valve 124d communicates with the ring-down chamber 84 toward its end adjacent to the front chamber mirror 88a, and the chamber outlet valve 124i communicates with the ring-down chamber 84 toward its end adjacent to the rear chamber mirror 88b.

[0055] When a new gaseous sample is to be loaded into the ring-down chamber 84 , the thermal desorption tube 104 containing the new gaseous sample is connected to the pneumatic system 112 , as shown.

[0056] During the evacuation phase, the vacuum intake valve 124k is opened and the vacuum pump 148 is turned on. The vacuum intake valve 124k is then closed, and the vacuum shut-off valve 124j, the chamber outlet valve 124i, the chamber inlet valve 124d, the inlet line valve 124c, and the access valve 124e are opened in sequence. The contents of the pipelines along the path and the ring-down chamber 84 are evacuated from the CRDS system 20 by the vacuum pump 148. The pressure gauge 144 is able to determine when the system is sufficiently evacuated, particularly when the pressure gauge 128 is disconnected from the vacuum pump 148. When it is determined that the system has been sufficiently evacuated, these same open valves 124j, 124i, 124d, 124c, and 124e are then closed in reverse order. Thereafter, during the nitrogen filling phase, valves 124a, 124c, 124d, 124i, and 124j are opened to allow nitrogen from the nitrogen source 116 to fill the pipelines 120a and 140. Another evacuation phase is then used to purge the nitrogen. The nitrogen filling phase and evacuation phase may be repeated as needed to purge the line. The CRDS system 20 is thus evacuated of the gaseous sample previously tested.

[0057] During loading of a new sample, the thermal desorption tube 104 is flushed to move carbon dioxide and water out of the thermal desorption tube 104, thereby minimizing the amount of carbon dioxide and water loaded into the ring-down chamber 84. To flush the thermal desorption tube 104, the gas inlet valve 124a, the gas inlet line valve 124c, and the back valve 124g are opened to provide a path to nitrogen gas to flush forward the thermal desorption tube 104. The thermal desorption tube 104 is selected to inhibit the collection of carbon dioxide and water with the sample, and some carbon dioxide and water are usually still present in the thermal desorption tube 104.

[0058] 500 ml of nitrogen is passed through the thermal desorption tube to remove the carbon dioxide and water remaining in the thermal desorption tube 104 from the original sample. Then 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 allows the nitrogen and the entrained carbon dioxide and water to be released at a specified flow rate. In this configuration, the flow rate is 500 ml / min. Then all valves 124 are closed.

[0059] Once the carbon dioxide and water are removed from the thermal desorption tube 104, the pneumatic system 112 is evacuated again using the same method described above to remove the nitrogen that was just introduced into the line of the pneumatic system 112. The heater 132 surrounding the thermal desorption tube 104 then heats the thermal desorption tube 104 to the desired temperature to thermally desorb the new sample in the thermal desorption tube 104. The gas inlet valve 124a, the passage valve 124e, the front valve 124f, the rear valve 124g, and the chamber inlet valve 124d are then opened to provide a direct path for nitrogen from the nitrogen source 116 through the thermal desorption tube 104 having the desorbed compound of interest to the ring-down chamber 84.

[0060] It is desirable to obtain a pressure of one atmosphere within the ring-down chamber 84 because all reference data collected and analyzed is at this pressure level, thereby ensuring that the results are repeatable.

[0061] The gas inlet valve 124a is triggered to open and close by the system, and then the system waits for the pressure reading at the pressure gauge 128 to stabilize and reach one atmosphere. If the pressure reading is still below one atmosphere when the pressure gauge 128 stabilizes, the gas inlet valve 124a is switched again to repeat the process until the pressure reading is one atmosphere. When the pressure gauge 128 shows that the pressure level in the ring-down chamber 84 is one atmosphere, the valves are all closed.

[0062] If desorption at multiple temperatures is desired, the vacuum pump 148 is turned on, the chamber outlet valve 124i and the vacuum shut-off valve 124j are opened to evacuate the ring-down chamber 84. The chamber outlet valve 124i is then closed before repeating the desorption process.

[0063] Since some gaseous sample may be lost between back valve 124g and chamber inlet valve 124d, full evacuation is not typically performed between desorptions.

[0064] By pressurizing a fixed volume ring-down chamber containing a gaseous sample to a desired pressure level in this manner, the surface area within the ring-down chamber to which compounds can adhere can be reduced compared to a variable volume ring-down chamber that can be used to raise the pressure within the chamber to a desired level.

[0065] Furthermore, the pressure gauge 128 is located upstream of the path of the gaseous sample from the thermal desorption tube 104 to the ring-down chamber 84, thereby preventing sample contamination.

[0066] Figure 2 is a schematic diagram of an electronic control subsystem 200 for various components of the CRDS system 20 also illustrated. All lines represent electrical or electronic signals, arrows represent one-way communications, voltage settings, etc., and lines without arrows represent two-way communications.

[0067] A computer 204 including one or more processors acts as a controller, which controls Figure 1 The functions of the various components are shown.

[0068] A pair of RF drivers 208 sends a 40MHz signal to power the CO2 laser 24 and the carbon 13O2 laser 28. Output couplers and actuators 212 are used to tune each of the lasers 24, 28. Each output coupler is driven by a 1000V output coupler piezo 216. The dual-channel high-voltage amplifier 220 that powers the output coupler piezo 216 can be adjusted between 0V and 1000V. The high-voltage amplifier 220 is provided with an analog output signal from a data acquisition ("DAQ") card 224 in the computer 204. The DAQ generates an output between 0V and 10V, and the high-voltage amplifier 220 multiplies the signal by 100 to generate a 0V to 1000V signal, thereby powering the output coupler piezo 216. Each actuator 212 that changes the grating angle is driven by an actuator driver 228, which is given instructions by the computer 204 via RS-232. Each actuator 212 moves a number of millimeters, which is translated into a pitch angle of the laser 24 , 28 .

[0069] Data signals from the pressure gauges 132, 144 of the pneumatic system 112 are received via RS-232.

[0070] The fast detector 56 is connected to a small amplifier 232 and an oscilloscope 236 which can be used to read the amplitude and frequency of the beat signal used to tune the lasers 24 , 28 .

[0071] The temperature controller 240 for the thermal desorption tube heater 132 is controlled by the computer 204 via RS-232. The tube heater 132 contains a temperature sensor and an aluminum sheet with a heating tape wrapped around it. Both the heating tape and the temperature sensor are connected to the temperature controller 240 which is a PID (Proportional Integral Derivative) controller. The controller sets and reads the temperature back to the host computer 204 via RS-232.

[0072] The relay board 244 is connected to the computer 204 and is used to open and close all the solenoid valves 124 and the vacuum pump 148 .

[0073] The three channel piezo driver 248 drives the piezo actuator 252 which actuates the micrometer 96b to adjust the length of the ring-down cavity 84. Each channel has two components: communication with the piezo driver via RS-232 and analog input from the DAQ card 224.

[0074] Each AOM 60, 68 is driven by an RF driver 256 that sends a signal of approximately 40 MHz. Changing the frequency of the RF driver 256 changes the Bragg angle for a given wavelength of light, or changes the wavelength of light to which a given or fixed Bragg angle is tuned. If the RF driver 256 is tuned to a specific frequency and set to full power, most of the working beam 52 (about 85%) passes through. If adjusted to 80%, 70%, the optical modulator 60, 68 will attenuate. If the RF driver 256 is set to zero, the optical modulator 60, 68 is completely turned off. The frequency of the RF driver is set via the RS232 pass component. Analog and digital components can set the amplitude and on / off conditions of the RF driver 256. In particular, the DAQ card 224 sends a signal to the timing circuit 260, which in turn generates the four necessary signals required to enable and set the amplitude of the RF driver. The timing circuit 260 can operate in a steady-state condition or a ring-down trigger condition, where the timing circuit 260 sets the four voltages to zero and then returns to the previous voltage level after a predetermined amount of time.

[0075] There is a digital output ("DO") from the DAQ card 224 that sets the timing circuit 260 to either a steady state or ring-down trigger mode. When in the ring-down trigger mode, the trigger output from the digitizer 264 triggers the timing circuit 260 to set the RF driver voltage to zero. In the steady state mode, the timing circuit 260 passes the trigger output (TRIG) from the DAQ card 224 to the digitizer 264 to synchronize the cavity scan (via the AWG) with the liquid nitrogen cooled detector 100. That is, when sweeping across the entire free spectral range of the ring-down cavity 84, the approximate voltage that sets the cavity piezoelectric to resonate can be determined.

[0076] When it is desired to make a ring-down measurement, one of the lasers 24, 28 is tuned using the fast detector 56 and the oscilloscope 236. Once the laser is tuned, the ring-down cavity is swept by sending a ramp signal generated by the DAQ card 224 to the piezoelectric driver 248, and the corresponding output beam intensity is then measured on the liquid nitrogen cooled detector 100. This provides a picture of where the resonance point is in the ring-down cavity 84 (e.g., 10 V).

[0077] The piezo driver is set to 10V via RS-232 and a small 1V-2V amplitude sine wave is sent from the DAQ card 224 which also reaches the piezo driver 248. This results in a sine wave from 10-12V reaching the piezo driver 248. The liquid nitrogen cooled detector 100 will show a back and forth sweep over the resonant point in the ring down cavity 84 and the center position is adjusted via RS-232 to move it to a higher or lower unit with equal time between the forward and backward sweeps. This is to tune the ring down cavity 84 to the resonant point.

[0078] Measure 500 ring downs and calculate the decay time for absorption coefficient calculation.

[0079] Once the gaseous sample is loaded into the ring-down cavity 84, one laser 24, 28 is tuned to a specific wavelength and its light is directed through the first optical modulator 60, reflected by the mirror 64, through the second optical modulator 68, and reflected by the mirror 72 into the ring-down chamber 80. The optical modulators 60, 68 slightly attenuate the working beam 52 to modulate its intensity.

[0080] When the working beam 52 reaches the front cavity mirror 88a, a portion of about 0.1% penetrates the front cavity mirror 88a and enters the ring-down cavity 84. The majority of the working beam, about 99.9%, is initially reflected back to the working laser 24 or 28 along the same path.

[0081] Initially, the ring-down cavity 84 is not illuminated. Light enters the ring-down cavity 84, and when most of the light in the ring-down cavity 84 is reflected between the two cavity mirrors 88, the amount of light or power in the ring-down cavity 84 begins to increase as further light is introduced from the outside via the working beam 52. A portion of the light leaks out through the cavity mirror 88. It takes a while to "fill" the ring-down cavity 84 with light. At this point, there is a balance between the incident light and the leakage. Once this balance is reached, the laser light 24, 28 is eliminated or prevented from entering the ring-down cavity 84 via the optical modulator 60, 68.

[0082] When the ring-down cavity 84 is in resonance and close to equilibrium (i.e., the amount of light leaking out through the cavity mirror 88 is equal to the amount of light entering from the working beam 52), there is destructive interference with the incident laser light, so that no or very little of the incident laser light is reflected by the front cavity mirror 88a. As a result, once the ring-down cavity 84 is in equilibrium, reflections of the portion of the working beam 52 directed toward the front cavity mirror 88a within the bandwidth of the ring-down cavity are substantially eliminated.

[0083] When the ring-down cavity is in equilibrium, a ring-down event can be initiated. Light entering the ring-down cavity 84 is eliminated as quickly as possible, and an infrared detector (i.e., a liquid nitrogen cooled detector 100) measures the light intensity leaving the back end of the ring-down cavity 84 to determine the exponential decay of the light intensity. It takes a certain amount of time for the light in the ring-down cavity 84 to decay or decay. The decay constant (T), which is defined as the length of time for the intensity to drop to 1 / e of the starting intensity (equal to approximately 0.37) or some other level, can be measured and then compared to the baseline decay time without the sample to determine how much light is absorbed by the gas sample. The acceleration in the ring-down is attributed to the presence of the gaseous sample in the ring-down cavity 84. Using the measured decay time, the absorption coefficient for frequency / wavelength can be calculated.

[0084] To eliminate the light entering the ring-down cavity 84, the computer 204 acts as a controller that directs the first optical modulator 60 to attenuate the light beam at or near the attenuation limit of the first optical modulator 60, and simultaneously directs the second optical modulator 68 to further attenuate the light beam at or near the attenuation limit of the second optical modulator 68 to reduce the intensity of the light beam from the first optical modulator 60. In a conventional CRDS system, the light deflected by a single optical modulator drops to zero over a short time span. Additional light allowed into the ring-down cavity 84 can skew the spectral result. Therefore, it is desirable to eliminate the laser as quickly as possible.

[0085] In the CRDS system 20, by directing both optical modulators 60, 68 to be turned off simultaneously, the amount of light deflected by the first optical modulator 60 during a relatively short time interval is significantly reduced by the second optical modulator 68 when the second optical modulator 68 is turned off.

[0086] The second optical module 68 greatly increases the attenuation achieved via the first optical modulator 60 alone. In the currently described embodiment, if the first optical modulator 60 can attenuate 30 dB, and the second optical modulator 68 can attenuate an additional 30 dB, the total attenuation achieved via the optical modulators 60, 68 is the sum of their attenuations, i.e., 60 dB. During the filling of the ring-down cavity 84 with light, the optical modulators 60, 68 attenuate the working beam 52 to modulate its intensity. In the present configuration, each optical modulator 60, 68 attenuates the working beam 52 by 5 dB, for a total attenuation of 10 dB. As a result, each of the optical modulators 60, 68 can also further attenuate the working beam 52 by 25 dB, for a total further attenuation of 50 dB. In a conventional setup, one optical modulator must attenuate the working beam by 10 dB, leaving 20 dB of further attenuation available to eliminate the working beam. As will be appreciated, the further attenuation of 50 dB via two optical modulators 60, 68 may extinguish the operating light beam 52 more quickly than one optical modulator with a further attenuation of 20 dB. As a result, the amount of additional light introduced into the ring-down cavity 84 after the optical modulators 60, 68 have been directed off is a fraction of the further light introduced by a single optical modulator arrangement in a conventional CRDS system. By extinguishing the operating light beam 52 more quickly, the optical attenuation measured in the ring-down cavity 84 is less affected by the additional light during the ramp-down time of the optical modulators 60, 68, thus providing greater accuracy when matching the observed decay time to the known decay time.

[0087] This process is repeated for multiple frequencies of light to generate an absorption spectrum of the gaseous sample. For example, the light generated by the CO2 laser 24 provides an absorption coefficient for a certain frequency range. Similarly, an absorption coefficient can be generated for a frequency range of light from the carbon 13O2 laser 28. In this way, an absorption spectrum of the sample can be generated.

[0088] Although in the above embodiments, the light sources are two lasers that generate light in the mid-infrared range, it should be understood that other light sources may be employed. For example, lasers that generate light in the visible spectrum or near-infrared lasers may be used. In addition, in some cases, the CRDS system may include only one laser, or three or more lasers to generate the working beam.

[0089] Electro-optic modulators can be used in place of acousto-optic modulators.

[0090] The AOM can be configured to shift the frequency of the working beam up or down. As long as the net frequency shift achieved by the AOM causes the frequency of the working beam to deviate significantly from the frequency of the working beam generated by the laser, such that the reflected light is outside the bandwidth of the generated laser light, the amount of interference between the reflected light and the generated working beam can be minimized.

[0091] In other embodiments, more than two optical modulators may be employed in a CRDS system to provide further cancellation capability to more quickly cancel the operating beam at the onset of a ring-down event.

[0092] In other embodiments, one or more focusing lenses may be used to mode match the ring-down cavity.

[0093] In other embodiments, analysis of gaseous samples may be performed at pressure levels other than one atmosphere. The width of the absorption spectrum may be altered accordingly.

[0094] While specific advantages are enumerated above, various embodiments may include some, none, or all of the enumerated advantages.

[0095] Those skilled in the art will appreciate that there are many more possible alternative implementations and modifications, and the above examples are only illustrations of one or more implementations. Therefore, the scope of the present disclosure is limited only by the appended claims.

[0096] Reference numerals list

[0097] 20 CRDS system

[0098] 24 CO2 laser

[0099] 28 Carbon 13O2 Laser

[0100] 32 First laser beam

[0101] 36 Second laser beam

[0102] 40 Reflector

[0103] 44 Beam Splitter

[0104] 48 Sampling beam

[0105] 52 Output beam

[0106] 56 Fast Detector

[0107] 60 First light modulator

[0108] 64 Reflector

[0109] 68 Second light modulator

[0110] 72 Focusing lens

[0111] 76 Reflector

[0112] 80 Ring-down Chamber

[0113] 84 Ring-down cavity

[0114] 88 Laparoscope

[0115] 88a Anterior cavity mirror

[0116] 88b Rear cavity mirror

[0117] 92 Reflector Mounting

[0118] 96 Micrometer

[0119] 100 Liquid nitrogen cooled detector

[0120] 104 Thermal Desorption Tube

[0121] 108 Receiver

[0122] 112 Pneumatic system

[0123] 116 Nitrogen source

[0124] 120 Catheter

[0125] 120a Gas input line

[0126] 120b Desorption line

[0127] 120c Sample output line

[0128] 124 Solenoid valve

[0129] 124a Gas inlet valve

[0130] 124b Auxiliary gas inlet valve

[0131] 124c Inlet line valve

[0132] 124d Cavity inlet valve

[0133] 124e Access valve

[0134] 124f Forward valve

[0135] 124g rear valve

[0136] 124h Cavity outlet valve

[0137] 124i Sample Outlet Valve

[0138] 124j Vacuum cut-off valve

[0139] 124k Vacuum Suction Valve

[0140] 128 Pressure gauge

[0141] 130a, 130b Filters

[0142] 132 Heater

[0143] 136 Mass Flow Controller

[0144] 140 Exit Line

[0145] 144 Pressure gauge

[0146] 148 Vacuum Pump

[0147] 200 Electronic control subsystem

[0148] 204 Computer

[0149] 208 RF Driver

[0150] 212 Actuator

[0151] 216 Output Coupler Piezoelectric

[0152] 220 High Voltage Amplifier

[0153] 224 DAQ Card

[0154] 228 Actuator Driver

[0155] 232 Amplifier

[0156] 236 Oscilloscope

[0157] 240 Temperature Controller

[0158] 244 Relay Board

[0159] 248 Three-Channel Piezoelectric Driver

[0160] 252 Piezoelectric Actuator

[0161] 256 RF Driver

[0162] 260 Timing Circuit

[0163] 264 Digitizer

Claims

1. A cavity ring-down spectroscopy system, comprising: at least one laser for generating a light beam; a first acousto-optic modulator positioned to attenuate a beam from the at least one laser; a second AOM positioned to attenuate the light beam from the first AOM; a ring-down cavity having two mirrors, the ring-down cavity being positioned to receive the light beam from the second AOM; as well as at least one light sensor for measuring the intensity of light leaking from the ring-down cavity; The first AOM and the second AOM are configured to collectively shift the frequency of the light beam to significantly deviate from the frequency of the light beam generated by the at least one laser so that the reflected light is outside the bandwidth of the generated light beam to minimize the amount of interference between the reflected light and the generated light beam.

2. The cavity ring-down spectroscopy system according to claim 1, further comprising at least one focusing lens for focusing the light beam to match the optical mode of the ring-down cavity.

3. The CRM system according to claim 2, further comprising a controller, wherein the controller is configured to guide the first AOM to attenuate the light beam at or close to the attenuation limit of the first AOM, and simultaneously guide the second AOM to further attenuate the light beam at or close to the attenuation limit of the second AOM, so as to reduce or eliminate the intensity of the light beam from the first AOM.

4. A method for modulating a light beam in a cavity ring-down spectroscopy system, comprising: generating a light beam via at least one laser; cancelling a beam from the at least one laser via a first acousto-optic modulator; canceling the light beam from the first AOM via a second AOM; receiving the light beam attenuated by the second AOM in the ring-down cavity; as well as measuring a decay rate of light leaking from the ring-down cavity; The first AOM and the second AOM are configured to collectively shift the frequency of the light beam to significantly deviate from the frequency of the light beam generated by the at least one laser so that the reflected light is outside the bandwidth of the generated light beam to minimize the amount of interference between the reflected light and the generated light beam.

5. The method according to claim 4, further comprising: directing the first AOM to attenuate the light beam at or near the attenuation limit of the first AOM; as well as The second AOM is simultaneously directed to further attenuate the light beam traveling toward the ring-down cavity at or near the attenuation limit of the second AOM to reduce or eliminate the intensity of the light beam from the first AOM.

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