Patent Description:
Cavity ring-down spectroscopy ("CRDS") is an approach that is generally used to identify and quantify a single analyte in a gaseous sample using their absorption spectra. A typical CRDS system employs a laser generating a beam that is directed into a cavity of a chamber having two highly reflective mirrors. The beam is normally within the visible light spectrum, often in the near infrared ("IR") spectrum, and is tuned to a single wavelength to identify the presence of a single molecule. The beam is then reflected repeatedly between the mirrors, which allow a fraction of the light to escape the ring-down cavity. When the laser is in resonance with a cavity mode, intensity builds up 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, when empty, decays at a pre-determined rate. A small fraction of the light is not reflected by the mirrors and escapes the ring-down cavity. The intensity of the escaping light is measured by a sensor component to determine the decay rate.

When the gaseous sample is placed in the ring-down cavity, analytes present in the gaseous sample absorb some of the light, thereby accelerating the decay of the intensity of the light in the ring-down cavity. Absorption spectra are generated by measuring the decay times of the light in the presence of the gaseous sample at specific wavelengths relative to the decay times of the light in the absence of the gaseous sample at these wavelengths. A linear regression of the measured absorption spectra for the gaseous sample with the known absorption spectra of various analytes enables the identification and quantification of individual analytes in the gaseous sample.

<NPL>, describe different light modulators used in cw-CRDS experiments and the use of a semiconductor optical amplifier (SOA) as light modulator in cw-CRDS. A direct comparison of the sensitivity realised on the same instrument using an SOA as modulator with use of an acousto-optic modulator (AOM) is described.

<CIT> describes a CRDS system including one or more acousto-optic modulators which may be electrically coupled with a triggering circuit controlling the operation of the one or more acousto-optic modulators, so as to extinguish light being delivered to a target volume so that the light decay in the target volume (or the ring-down signal) can be measured, wherein the one or more acousto-optic modulators may have a fast switching (extinction) time (e.g., less than about <NUM> microsecond).

<NPL>, discloses cavity ringdown spectroscopy achieving high spectral resolution with tunable narrow bandwidth pulsed lasers, wherein two acousto-optic modulators having opposite diffraction orders may be arranged in series.

<NPL>, discloses design, construction, and initial performance evaluation of a high-repetition-rate cavity ringdown spectrometer, wherein an AOM and an EOM are arranged in series.

The present invention is defined in claims <NUM> and <NUM>.

In one aspect, there is provided a cavity ring-down spectroscopy system according to claim <NUM>.

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

The first and second optical modulators can be acousto-optic modulators.

In another aspect, there is provided a method of modulating a light beam in a cavity ring-down spectroscopy system according to claim <NUM>.

Other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.

For a better understanding of the embodiment(s) described herein and to show more clearly how the embodiment(s) may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which:.

Unless otherwise specifically noted, articles depicted in the drawings are not necessarily drawn to scale.

For simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the Figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiment or 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. It should be understood at the outset that, although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described below.

Various terms used throughout the present description may be read and understood as follows, unless the context indicates otherwise: "or" as used throughout is inclusive, as though written "and/or"; singular articles and pronouns as used throughout include their plural forms, and vice versa; similarly, gendered pronouns include their counterpart pronouns so that pronouns should not be understood as limiting anything described herein to use, implementation, performance, etc. by a single gender; "exemplary" should be understood as "illustrative" or "exemplifying" and not necessarily as "preferred" over other embodiments. Further definitions for terms may be set out herein; these may apply to prior and subsequent instances of those terms, as will be understood from a reading of the present description.

As used in this document, "each" refers to each member of a set or each member of a subset of a set.

Any module, unit, component, server, computer, terminal, engine or device exemplified herein that executes instructions may include or otherwise have access to computer readable media such as storage media, computer storage media, or data storage devices (removable and/or non-removable) such as, for example, magnetic disks, optical disks, or tape. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for 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 or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by an application, module, or both. Any such computer storage media may be part of the device or accessible or connectable thereto. Further, unless the context clearly indicates otherwise, any processor or controller set out herein may be implemented as a singular processor or as a plurality of processors. The plurality of processors may be arrayed or distributed, and any processing function referred to herein may be carried out by one or by a plurality of processors, even though a single processor may be exemplified. Any method, application or module herein described may be implemented using computer readable/executable instructions that may be stored or otherwise held by such computer readable media and executed by the one or more processors.

CRDS systems typically include an optical modulator to modulate light from a laser. The optical modulator may deflect the light to another path in order to attenuate the intensity or power of the light. Acousto-optic modulators ("AOMs") are one 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 electric signal is applied to the piezoelectric transducer, the piezoelectric transducer vibrates, creating sound waves in the material. These sound waves expand and compress the material, thereby creating periodic variations in the refractive index and allowing for Bragg diffraction. Light entering the AOM at the first order Bragg angle relative to the plane perpendicular to the axis or propagation of the acoustic wave will be deflected by an amount equal to twice the Bragg angle at maximum efficiency. Extinguishing the electric signal removes the Bragg diffraction properties of the material and causes the light to pass through undeflected, effectively attenuating the light along the deflected optical path. A by-product of the AOM is that the frequency of the light being deflected is shifted.

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 altered to change the phase of the outgoing beam as a function of the applied field. By sending the beam through a polarizer, the phase modulation is converted to intensity modulation. In another method, a phase modulator when placed in a branch of an interferometer can act as an intensity modulator.

The optical modulators are used to control the intensity of the light beam generated by the laser. One side effect of the use of acousto-optic modulators is that the frequency of the light is shifted. This shift is small relative to the absolute frequency of the light.

In CRDS, the rate of decay of light in the ring-down cavity is determined in order to understand the absorption spectrum(s) of the gaseous sample in the ring-down cavity. Traditional CRDS systems employing a single optical modulator, however, have difficulties extinguishing light supplied to the ring-down cavity as quickly and completely as desirable. This can be more true for mid-IR than for near-IR and visible wavelengths. Generally, an optical modulator does not operate to generate perfect step function output, and has ramp up and ramp down times. As a result, the additional light entering the ring-down cavity at the start of a ring-down event can be difficult to compensate for, making linear regression with known analyte absorption spectra more challenging.

Various components of a CRDS system <NUM> in accordance with a particular embodiment are shown in <FIG>. A CO<NUM> laser <NUM> and a carbon-<NUM> O<NUM> laser <NUM> are provided. The CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM> are gas tube lasers that emit at a series of quasi-evenly-spaced, well-known frequencies that can be rapidly selected using an adjustable diffraction grating apparatus. Gas tube laser technology has a long history and is a stable and robust way of generating infrared radiation at precisely-known frequencies. Both the CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM> emit light in the mid-IR spectrum.

Each of the CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM> has an actuator and an output coupler that enable adjustment of the length of the laser cavity as well as change the angle of grating at the back of the cavity, thereby changing its pitch to adjust which wavelengths it reflects. By both 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.

The CO<NUM> laser <NUM> produces a first laser beam <NUM>, and the carbon-<NUM> O<NUM> laser <NUM> produces a second laser beam <NUM>. Depending on the light frequency desired, either the CO<NUM> laser <NUM> is tuned and generates the first laser beam <NUM> while the carbon-<NUM> O<NUM> laser <NUM> is detuned, or the carbon-<NUM> O<NUM> laser <NUM> is tuned and generates the second laser beam <NUM> while the CO<NUM> laser <NUM> is detuned. In this manner, at most only one of the CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM> outputs a beam at any particular time so that the first beam <NUM> and the second beam <NUM> are not combined simultaneously. Mid-infrared, and specifically long wavelength infrared, was chosen as the type of light as most volatile organic compounds absorb light in this range. As a result, multiple volatile organic compounds can be measured by a single system. CO2 lasers operate in this range and have sufficient power and linewidth narrowness for ring-down spectroscopy. Using two lasers adds to the range and number of available wavelengths that the CRDS system <NUM> can use to analyze gaseous samples.

The first laser beam <NUM> is redirected via a mirror <NUM> on an optic mount towards a beam splitter <NUM>. The beam splitter <NUM> is partially reflective and partially transmissive, and splits each of the first laser beam <NUM> and the second laser beam <NUM> into two beams, a sampling beam <NUM>, and a working beam <NUM> that has the same characteristics as the sampling beam <NUM> and can be of similar intensity as the sampling beam <NUM>.

The sampling beam <NUM> is received by a fast detector <NUM>. The fast detector <NUM> measures the amplitude and the beat frequency of the sampling beam <NUM> using an oscilloscope. The beat frequency can indicate the presence of higher order modes resulting from a less-than-optimal tuning of the CO<NUM> laser <NUM> or the carbon-<NUM> O<NUM> laser <NUM>. In response to the detection of an undesirable beat frequency, the corresponding laser <NUM> or <NUM> 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 can be tuned to a different wavelength.

The working beam <NUM> continues to a first optical modulator <NUM>, which then deflects the working beam <NUM> to a mirror <NUM> on an optic mount. The mirror <NUM> redirects the light towards a second optical modulator <NUM> that, in turn, deflects the working beam <NUM> to a focusing lens <NUM>. In the present embodiment, the first and second optical modulators <NUM>, <NUM> are AOMs, also referred to as Bragg cells, but could also be electro-optic modulators in other embodiments.

The first and second optical modulators <NUM>, <NUM> act as attenuators to adjust the intensity of the working beam <NUM> and extinguish the beam at the commencement of a ring-down event. As they are AOMs, the first and second optical modulators <NUM>, <NUM> use the acousto-optic effect to diffract the light using sound waves (normally at radio-frequency). 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 electric signal is used to cause the piezoelectric transducer to vibrate. The vibrating piezoelectric transducer creates sound waves in the material that expand and compress the material, thereby creating period variations in the refractive index and allowing for Bragg diffraction. Light entering the AOM at Bragg angle relative to the 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. Extinguishing the electric signal removes the Bragg diffraction properties of the material and causes the light to pass through undeflected, effectively extinguishing the light along the deflected optical path. Hence, the intensity of the sound can be used to modulate the intensity of the light in the deflected beam.

The intensity of the light deflected by each of the first and second optical modulators <NUM>, <NUM> can be between about <NUM>%, representing a maximum deflection efficiency of the optical modulators <NUM>, <NUM>, and an attenuation limit of each of the first and second optical modulators <NUM>, <NUM> of about <NUM>% of the input light intensity. When the acoustic wave applied to the germanium is turned off, the deflected beam loses about <NUM> dB, or <NUM>%, of the previous intensity. The attenuation limit means the upper limit of how much of the input light intensity can be reduced by the optical modulator.

Optic modulators are asymmetrical in that, as a side effect, they Doppler-shift the frequency of light in a first mode when the input light is received at a first end thereof, and they Doppler-shift the frequency of light in a second mode that is counter to the first mode when the input light is received at a second end thereof and the attenuation power is the same. The Doppler shift of the frequency of the light is, however, in the same direction regardless of whether the light enters at a first end or at a second end.

Conventional CRDS systems use a single optical modulator and, as a result, have a working beam that is frequency shifted. These frequency shifts are generally small in relation to the frequency of the light, and can change the manner in which the light is absorbed by matter in the cavity, but this frequency shift can be compensated for during the analysis. If diffraction is towards the acoustic wave source of an AOM, the frequency shift is downwards, and if diffraction is away from the acoustic wave source, the frequency shift is upwards. As discussed, the effect is minimal.

The working beam <NUM> deflected by the second optical modulator <NUM> is focused via a focusing lens <NUM> to match an optical mode of the ring-down cavity <NUM>. As the laser beam, and thus the working beam <NUM>, travels from the CO<NUM> laser <NUM> or the carbon-<NUM> O<NUM> laser <NUM>, it continues to diverge. The focusing lens <NUM> focuses the working beam <NUM> back down.

A mirror <NUM> on an optic mount thereafter redirects the working beam <NUM> towards a ring-down chamber <NUM>. The two mirrors <NUM>, <NUM> extend the length of the path of the working beam <NUM>.

The ring-down chamber <NUM> is an elongated tube defining a ring-down cavity <NUM> therein. A front cavity mirror 88a and a rear cavity mirror 88b (alternatively referred to herein as cavity mirrors <NUM>) are positioned at longitudinal ends of the ring-down cavity <NUM>. The cavity mirrors <NUM> are highly reflective, both to light directed to the cavity mirrors <NUM> from outside of the ring-down cavity <NUM> and directed to the cavity mirrors <NUM> within the ring-down cavity <NUM>. As a result, a fraction of the working beam <NUM> is directed at the front cavity mirror 88a, about <NUM>%, passes through the front cavity mirror 88a, and enters the ring-down cavity <NUM>, and the majority of the working beam <NUM>, about <NUM>% is reflected back towards the mirror <NUM>.

The cavity mirrors <NUM> are mounted on mirror mounts <NUM> that are actuatable to adjust the positioning and orientation of the cavity mirrors <NUM>. In particular, the front cavity mirror 88a towards the front of the ring-down cavity <NUM> is mounted on a mirror mount <NUM> that is actuatable via three mechanized micrometers 96a. The rear cavity mirror 88b towards the rear of the ring-down cavity <NUM> is mounted on a mirror mount <NUM> that is actuatable via three motorized micrometers 96b that can be manually adjusted for optical alignment or with a piezo that allows them to be adjusted further with the piezo driver.

The angle of each of cavity mirror <NUM> can be changed so that they are perfectly aligned so that when a light beam enters the ring-down cavity <NUM>, the light beam does not deviate. If one of the cavity mirrors <NUM> is askew, then some of the light gets reflected to the side of the ring-down cavity <NUM>, intensity of the light is lost, high-order modes result, amongst other things. The micrometers <NUM> can also be simultaneously tuned to change the length of the ring-down cavity <NUM>. This allows for the tuning of the ring-down cavity <NUM> so that the ring-down cavity <NUM> resonates at the frequency of the light that is entering the ring-down cavity <NUM>.

The focusing lens <NUM> focuses the laser light to match the optical mode of the ring-down cavity <NUM>, so that the minimum waist of the beam is positioned at the same place as the minimum beam waist of the ring-down cavity <NUM>.

A light sensor in the form of a liquid nitrogen-cooled detector <NUM> is positioned behind the rear cavity mirror 88b to receive light escaping through it. The liquid nitrogen-cooled detector <NUM> measures the intensity of the light that escapes the ring-down cavity <NUM>. Other types of sensors for measuring the intensity of the escaping light can be used in place of the liquid nitrogen-cooled detector <NUM>, such as a thermo-electrically cooled detector.

During a tuning process, one of the CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM> is tuned and the sampling beam <NUM> reaching the fast detector <NUM> is analyzed to identify a beat frequency. If a beat frequency is present in the sampling beam <NUM>, the corresponding laser is adjusted until the undesirable beat frequency is removed or diminished in amplitude below an acceptable limit.

In addition, the position of the cavity mirrors <NUM> are adjusted via the micrometers <NUM> so that no higher order modes are present in the ring-down cavity <NUM>.

Gaseous samples are loaded into the ring-down cavity <NUM> from a thermal desorption tube <NUM> that is used to collect the gaseous samples for testing. Thermal desorption tubes are generally made of stainless steel and contain various types of solid adsorbent material. The solid sorbents are selected for sampling specific compounds to trap and retain the compounds of interest even in the presence of other compounds, and allow the collected compounds to be easily desorbed or extracted for analysis. In addition, the solid sorbents which are selected do not react with the compounds of interest.

In a particular example, the gaseous samples are human breath samples collected from patients. A receiving end <NUM> of the thermal desorption tube <NUM> receives human breath collected from a human for testing. As a result, compounds of interest are more concentrated towards the receiving end <NUM> of the thermal desorption tube <NUM>.

A pneumatic system <NUM> is used to load gaseous samples from thermal desorption tubes <NUM> into the ring-down cavity <NUM>, and evacuate the pneumatic system <NUM>, including the ring-down cavity <NUM>. During loading of a gaseous sample, the pneumatic system <NUM> fills the ring-down cavity <NUM> with the gaseous sample that has been collected (i.e., to desorb the gaseous sample from the thermal desorption tube <NUM>, get the gaseous sample into the ring-down cavity <NUM> without introducing contaminants), brings the pressure and temperature in the ring-down cavity to one atmosphere and <NUM> degrees Celsius, and seals the ring-down cavity <NUM>. In this embodiment, the absorption spectra for 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. In other embodiments, however, the pressure and temperature can be fixed at other levels for the known and measured absorption spectra. During evacuation of a gaseous sample, the pneumatic system <NUM> cleans the previously provided gaseous sample from the ring-down cavity <NUM> and the various conduits for guiding gaseous samples from the thermal desorption tube <NUM> to the ring-down cavity <NUM>.

The pneumatic system <NUM> has an intake portion that includes a nitrogen gas source <NUM>. The nitrogen gas source <NUM> is a supply of very clean nitrogen gas that is pressurized or that can pressurize the nitrogen gas to at least above one atmosphere of pressure. In the present embodiment, the nitrogen gas source <NUM> is pressurized at five psi above ambient pressure, but can be varied as long as the compression is sufficient to pressurize the ring-down cavity <NUM> to one atmosphere, or some other selected atmospheric pressure at which the analyses are run. In the illustrated embodiment, the nitrogen gas source <NUM> is the nitrogen gas that evaporates off a liquid nitrogen reservoir. The nitrogen gas source <NUM> is connected via conduit <NUM> to a gas inlet valve 124a. An auxiliary gas inlet valve 124b enables connection of other gases, but is not regularly employed. The gas inlet and auxiliary gas inlet valves 124a, 124b are in communication with a gas intake line 120a. A pressure meter <NUM> is positioned along the gas intake line 120a, as well as a gas intake line valve 124c. A filter 130a is positioned along the gas intake line 120a in front of a cavity inlet valve 124d that seals the gas intake line 120a from the ring-down cavity <NUM>. The filter 130a inhibits the entry of contaminants into the ring-down cavity <NUM> where they can deposit on the cavity mirrors <NUM> and interfere with reflection.

The gas inlet and auxiliary gas inlet valves 124a, 124b are in communication with a pathing valve 124e. The pathing valve 124e enables or disables direct access to a desorption tube line 120b and a sample outlet line 120c.

The desorption tube line 120b includes a forward valve 124f and a rearward valve <NUM>. The thermal desorption tube <NUM> is positioned between the forward valve 124f and the rearward valve <NUM>, with the receiving end <NUM> of the thermal desorption tube <NUM> being positioned towards the rearward valve <NUM>. The thermal desorption tube <NUM> is positioned within a heater <NUM>.

The sample outlet line 120c includes a sample outlet valve <NUM> and a mass flow controller <NUM>.

The pneumatic system <NUM> also has an outlet portion that includes a cavity outlet valve 124i in communication with the ring-down cavity <NUM>. An outlet line <NUM> is in communication with the cavity outlet valve 124i. A pressure meter <NUM> is positioned along the outlet line <NUM>. A vacuum cutoff valve 124j is positioned between the pressure meter <NUM> and a vacuum pump <NUM>. A vacuum intake valve <NUM> is in communication with the vacuum pump <NUM> and draws air through a pump intake line <NUM>. A filter 130b is positioned in the pump intake line <NUM> to inhibit entry of contaminants that can interfere with the working of the vacuum pump <NUM>.

Valves 124a to <NUM> may be alternatively referred to herein as valves <NUM>.

While the cavity inlet valve 124d and the cavity outlet valve 124i are shown for convenience coupled to the ring-down cavity <NUM> at certain locations, it will be understood that the locations at which the valves 124d, 124i are coupled to the ring-down cavity <NUM> may vary. In a preferred configuration, the cavity inlet valve 124d is in communication with the ring-down cavity <NUM> towards an end thereof adjacent the front cavity mirror 88a and the cavity outlet valve 124i is in communication with the ring-down cavity <NUM> towards an end thereof adjacent the rear cavity mirror 88b.

When a new gaseous sample is to be loaded into the ring-down cavity <NUM>, the thermal desorption tube <NUM> containing the new gaseous sample is coupled to the pneumatic system <NUM> as shown.

During an evacuation phase, the vacuum intake valve <NUM> is opened and the vacuum pump <NUM> is turned on. The vacuum intake valve <NUM> is then closed, and the vacuum cutoff valve 124j, the cavity outlet valve 124i, the cavity inlet valve 124d, the gas intake line valve 124c, and the pathing valve 124e are opened in succession. The contents of the lines along this path and the ring-down cavity <NUM> are evacuated from the CRDS system <NUM> by the vacuum pump <NUM>. The pressure meter <NUM> enables the determination of when the system has been evacuated sufficiently, especially when the pressure meter <NUM> is cut off from the vacuum pump <NUM>. When it is determined that the system has been evacuated sufficiently, these same open valves 124j, 124i, 124d, 124c, and 124e are then closed in the reverse order. Thereafter, during a nitrogen fill phase, valves 124a, 124c, 124d, 124i, and 124j are opened to allow nitrogen gas from the nitrogen gas source <NUM> to fill the lines 120a and <NUM>. The nitrogen gas is then purged using another evacuation phase. The nitrogen fill phase and the evacuation phase can be repeated as desired to clear out the lines. The CRDS system <NUM> is thus evacuated of the previously tested gaseous sample.

During the loading of the new sample, the thermal desorption tube <NUM> is flushed to remove carbon dioxide and water out of the thermal desorption tube <NUM> so that the amount of carbon dioxide and water loaded into the ring-down cavity <NUM> is minimized. In order to flush the thermal desorption tube <NUM>, the gas intake valve 124a, the gas intake line valve 124c, and the rearward valve <NUM> are opened to give a path to the nitrogen gas to forward flush the thermal desorption tube <NUM>. The thermal desorption tube <NUM> is selected to inhibit the collection of carbon dioxide and water with the gaseous sample, but there is still typically some carbon dioxide and water in the thermal desorption tube <NUM>.

<NUM> of nitrogen gas is put through the thermal desorption tube to get out carbon dioxide and water that have remained in the thermal desorption tube <NUM> from the original sample. Then the forward valve 124f and the sample outlet valve <NUM> are opened to provide a path to the mass flow controller <NUM>. The mass flow controller <NUM> allows the nitrogen gas and borne carbon dioxide and water to be released at a specified flow rate. In the present configuration, this flow rate is <NUM> / min. All the valves <NUM> are then closed.

Once the carbon dioxide and the water have been removed from the thermal desorption tube <NUM>, the pneumatic system <NUM> is evacuated again using the same process discussed above to remove the nitrogen gas just introduced in the pneumatic system <NUM> lines. The heater <NUM> surrounding the thermal desorption tube <NUM> then heats the thermal desorption tube <NUM> to the desired temperature to thermally desorb the new sample within the thermal desorption tube <NUM>. The gas inlet valve 124a, the pathing valve 124e, the forward valve 124f, the rearward valve <NUM>, and the cavity inlet valve 124d are then opened to provide a direct path for the nitrogen gas from the nitrogen gas source <NUM>, through the thermal desorption tube <NUM> having desorbed compounds of interest, and to the ring-down cavity <NUM>.

It is desired to achieve a pressure of one atmosphere within the ring-down cavity <NUM> as all of the reference data collected and analyzed is at this pressure level, thereby ensuring that the results are repeatable.

The gas inlet valve 124a is toggled open and closed by the system, then the system waits for the pressure reading at the pressure meter <NUM> to stabilize and reach one atmosphere. If, upon stabilization of the pressure meter <NUM>, the pressure reading is still below one atmosphere, the gas inlet valve 124a is toggled again to repeat the process until the pressure reading is one atmosphere. When the pressure meter <NUM> shows that the pressure level in the ring-down cavity <NUM> is one atmosphere, the valves are all closed.

If it is desired to desorb at multiple temperatures, the vacuum pump <NUM> is turned on, the cavity outlet valve 124i and the vacuum cutoff valve 124j are opened to evacuate the ring-down cavity <NUM>. Then the cavity outlet valve 124i is closed before the desorption process is repeated.

A full evacuation is generally not performed between multiple desorptions as there is still some of the gaseous sample between the rearward valve <NUM> and the cavity inlet valve 124d that would be otherwise lost.

By pressurizing a fixed volume ring-down cavity containing the gaseous sample to a desired pressure level in this manner, the surface area within the ring-down cavity to which compounds can adhere can be decreased in comparison to variable volume ring-down cavities that may be used to raise the pressure within the cavity to the desired level.

Further, the pressure meter <NUM> is upstream from the path of the gaseous sample from the thermal desorption tube <NUM> to the ring-down cavity <NUM>, thereby preventing its contamination by the sample.

<FIG> is a schematic diagram of an electronic control subsystem <NUM> for various components of the CRDS system <NUM> that are also illustrated. All of the lines represent electrical or electronic signals, with arrows representing unidirectional communications, setting of a voltage, etc., and lines that are not arrows representing bidirectional communications.

A computer <NUM> including one or more processors acts as a controller that controls the function of the various components illustrated in <FIG>.

A pair of RF drivers <NUM> send <NUM> signal to power the CO<NUM> laser <NUM> and the carbon-<NUM> O<NUM> laser <NUM>. Each of the lasers <NUM>, <NUM> is tuned using an output coupler and an actuator <NUM>. Each output coupler driven by a 1000V output coupler piezo <NUM>. A two-channel high-voltage amplifier <NUM> that powers the output coupler piezos <NUM> is adjustable between 0V and 1000V. The high-voltage amplifier <NUM> is set with an analog output signal from a data acquisition ("DAQ") card <NUM> in the computer <NUM>. The DAQ generates output between 0V and 10V, and the high-voltage amplifier <NUM> multiplies the signal by <NUM> to generate a signal of 0V to 1000V to power the output coupler piezo <NUM>. Each actuator <NUM> that changes the angle for the grating is driven by an actuator driver <NUM> that is given instructions by the computer <NUM> via RS-<NUM>. Each actuator <NUM> is moved so many millimeters, which is translated into a pitch angle of the laser <NUM>, <NUM>.

Data signals from the pressure meters <NUM>, <NUM> of the pneumatic system <NUM> are received through RS-<NUM>.

The fast detector <NUM> is connected to a small amplifier <NUM> and an oscilloscope <NUM> that can be used to read the amplitude and frequency of the beat signal that is used to tune the lasers <NUM>, <NUM>.

A temperature controller <NUM> for the thermal desorption tube heater <NUM> is controlled via RS-<NUM> by the computer <NUM>. The tube heater <NUM> includes a temperature sensor and a piece of aluminum that has heating tape wrapped around it. The heating tape and the temperature sensor are both connected to the temperature controller <NUM> which is a PID (proportional integral derivative) controller. The controller sets and reads back the temperature via RS-<NUM> to the main computer <NUM>.

A relay board <NUM> is connected to the computer <NUM> and is used to turn on and off all of the solenoid valves <NUM> and the vacuum pump <NUM>.

A three-channel piezo driver <NUM> drives the piezo actuators <NUM> that actuate the micrometers 96b to adjust the length of the ring-down cavity <NUM>. Each channel has two components: communications to the piezo driver through RS-<NUM>, and analog input from the DAQ card <NUM>.

Each acousto-optic modulator <NUM>, <NUM> is driven with an RF driver <NUM> that sends approximately a <NUM> signal. Changing the frequency of the RF driver <NUM> changes the Bragg angle for a given optical wavelength, or changes the optical wavelength that a given or fixed Bragg angle is attuned to. If the RF driver <NUM> is tuned to a specific frequency and set to full power, most of the working beam <NUM> (about <NUM>%) gets through. If adjusted to <NUM>%, <NUM>%, then the optical modulator <NUM>, <NUM> will attenuate. If the RF driver <NUM> is set to zero, the optical modulator <NUM>, <NUM> shuts off completely. The frequency of the RF driver is set through a component via RS232. An analog and digital component can set the amplitude and the on/off condition of the RF driver <NUM>. In particular, the DAQ card <NUM> sends a signal to the timing circuit <NUM> which, in turn, generates the four necessary signals needed to enable and set the amplitude of the RF drivers. The timing circuit <NUM> can operate in a steady state condition or a ring-down triggering condition where the timing circuit <NUM> sets the four voltages to zero, and then returns to the previous voltage level after a pre-determined amount of time.

There is a digital output ("DO") from the DAQ card <NUM> that sets the timing circuit <NUM> to either a steady state or ring-down triggering mode. When in ring-down triggering mode, a trigger out from a digitizer <NUM> triggers the timing circuit <NUM> to set the RF driver voltages to <NUM>. In steady state mode, the timing circuit <NUM> passes a trigger out (TRIG) from the DAQ card <NUM> to the digitizer <NUM> in order to synchronize cavity sweeps (through AWG) with the liquid nitrogen-cooled detector <NUM>. That is, when the entire free spectral range of the ring-down cavity <NUM> is swept, the approximate voltage to set the cavity piezos to in order to resonate can be determined.

When it is desired to do a ring-down measurement, one of the lasers <NUM>, <NUM> is tuned using the fast detector <NUM> and the oscilloscope <NUM>. once the laser is tuned, the ring-down cavity is swept by sending a ramp signal generated by the DAQ card <NUM> to the piezo driver <NUM>, and then the corresponding output beam intensity is measured on the liquid nitrogen-cooled detector <NUM>. This provides a picture of where the resonant point is in the ring-down cavity <NUM> (e.g., 10V).

The piezo driver is set to 10V via RS-<NUM>, and a small 1V-2V amplitude sine wave is sent from the DAQ card <NUM>, which also goes to the piezo driver <NUM>. This results in a sine wave from <NUM>-12V that goes to the piezo driver <NUM>. The liquid nitrogen-cooled detector <NUM> will show sweeping back and forth over the resonance point in the ring-down cavity <NUM>, and the center position is adjusted by RS-<NUM> to move it higher or lower unit the time between sweeping forward and backward is equal. This is to tune the ring-down cavity <NUM> to the resonance point.

<NUM> ring-downs are measured and the decay time which is used in the absorption coefficient calculation is calculated.

Once the gaseous sample is loaded in the ring-down cavity <NUM>, one laser <NUM>, <NUM> is tuned to a specific wavelength and its light is directed through the first optical modulator <NUM>, reflected by the mirror <NUM>, through the second optical modulator <NUM>, and reflected by the mirror <NUM> to the ring-down chamber <NUM>. The optical modulators <NUM>, <NUM> attenuate the working beam <NUM> somewhat to modulate its intensity.

When the working beam <NUM> reaches the front cavity mirror 88a, a fraction, about <NUM> %, penetrates the front cavity mirror 88a to enter the ring-down cavity <NUM>. The majority of the working beam, about <NUM>%, is initially reflected back along the same path to the working laser <NUM> or <NUM>.

Initially, the ring-down cavity <NUM> is not illuminated. Light enters the ring-down cavity <NUM> and, as the majority of the light in the ring-down cavity <NUM> is reflected between the two cavity mirrors <NUM>, the amount, or power, of light in the ring-down cavity <NUM> starts increasing as further light is introduced from outside via the working beam <NUM>. A certain fraction of the light leaks out past the cavity mirrors <NUM>. It takes a period of time to "fill" the ring-down cavity <NUM> with light. At that point, there is an equilibrium between the incoming light and the leakage. Once this equilibrium is achieved, the laser <NUM>, <NUM> is extinguished or otherwise stopped from entering the ring-down cavity <NUM> via the optical modulators <NUM>, <NUM>.

When the ring-down cavity <NUM> is in resonance and approaches equilibrium (that is, the amount of light leaking out via the cavity mirrors <NUM> is equal to the amount of light entering from the working beam <NUM>), there is destructive interference with the incoming laser light such that none or very little of the incoming laser light is reflected by the front cavity mirror 88a. As a result, once the ring-down cavity <NUM> is at equilibrium, reflection of the portion of the working beam <NUM> within the bandwidth of the ring-down cavity that is directed at the front cavity mirror 88a is substantially eliminated.

When the ring-down cavity is in equilibrium, a ring-down event can be started. The light entering the ring-down cavity <NUM> is extinguished as quickly as possible, and the infrared detector (that is, the liquid nitrogen-cooled detector <NUM>) measures light intensity exiting the back end of the ring-down cavity <NUM> to determine exponential decay in the intensity of the light. It takes a certain amount of time for the light in the ring-down cavity <NUM> to ring down or decay. A decay constant (T) defined as the length of time for the intensity to drop to <NUM>/e (equal to approximately <NUM>) of the starting intensity or some other level can be measured and then compared to a baseline decay time without the sample to determine how much light is being absorbed by the gaseous sample. The acceleration in the ring down is attributed to the presence of the gaseous sample in the ring-down cavity <NUM>. Using the measured decay times, an absorption coefficient can be calculated for the frequency / wavelength.

In order to extinguish the light entering the ring-down cavity <NUM>, the computer <NUM> acts as a controller that directs the first optical modulator <NUM> to attenuate the light beam at or close to an attenuation limit of the first optical modulator <NUM> and to simultaneously direct the second optical modulator <NUM> to further attenuate the light beam at or close to an attenuation limit of the second optical modulator <NUM> to reduce an intensity of the light beam from the first optical modulator <NUM>. In conventional CRDS systems, the light that is deflected by the single optical modulator drops off to zero over a short span of time. The additional light allowed to enter the ring-down cavity <NUM> can skew the spectroscopy results. It can therefore be desirable to have the laser light extinguished as quickly as possible.

In the CRDS system <NUM>, by directing both optical modulators <NUM>, <NUM> to shut off simultaneously, the amount of light deflected by the first optical modulator <NUM> during the short span of time is markedly reduced by the second optical modulator <NUM> as it is shutting down.

The second optical modulator <NUM> greatly increases the attenuation achieved via the first optical modulator <NUM> alone. In the currently described embodiment, if the first optical modulator <NUM> can attenuate by <NUM> dB, and the second optical modulator <NUM> can attenuate by an additional <NUM> dB, with the total attenuation achieved via the optical modulators <NUM>, <NUM> being the sum of their attenuation, or <NUM> dB. During filling of the ring-down cavity <NUM> with light, the optical modulators <NUM>, <NUM> attenuate the working beam <NUM> to modulate its intensity. In the present configuration, each of the optical modulators <NUM>, <NUM> attenuate the working beam <NUM> by <NUM> dB, for a total attenuation of <NUM> dB. As a result, each of the optical modulators <NUM>, <NUM> can still further attenuate the working beam <NUM> by <NUM> dB for a total further attenuation of <NUM> dB during the extinguishing of the working beam <NUM>. In a conventional setup, one optical modulator would have to attenuate a working beam by <NUM> dB, leaving <NUM> dB of further attenuation available for extinguishing the working beam. As will be understood, the working beam <NUM> can be extinguished much more rapidly via <NUM> dB of further attenuation via the two optical modulators <NUM>, <NUM> than with one optical modulator with <NUM> dB of further attenuation. As a result, the amount of additional light introduced into the ring-down cavity <NUM> after the optical modulators <NUM>, <NUM> have been directed to shut down is a small fraction of the light further introduced by a single optical modulator setup in a conventional CRDS system. By extinguishing the working beam <NUM> more quickly, the measured decay of light in the ringdown cavity <NUM> is less affected by the additional light during the ramp-down times of the optical modulators <NUM>, <NUM>, thus granting higher precision when matching the observed decay times against known decay times.

The process is repeated for lights of multiple frequencies to generate an absorption spectrum for the gaseous sample. For example, the light generated by the COz laser <NUM> provides absorption coefficients for a range of frequencies. Similarly, absorption
coefficients can be generated for a range of frequencies for the light from the carbon-<NUM> O<NUM> laser <NUM>. In this manner, an absorption spectrum can be developed for the sample.

While, in the above-described embodiment, the light sources are two lasers that produce light in the mid-infrared range, it will be appreciated that other light sources can be employed. For example, a laser producing light in the visible spectrum or a near-infrared laser can be employed. Further, in some scenarios, the CRDS system can include only one laser, or three or more lasers, to generate the working beam.

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

The acousto-optic modulators can be configured so that the frequency of the working beam is shifted up or down. As long as the net frequency shift effected by the acousto-optic modulators shifts the frequency of the working beam significantly away from the frequency of the working beam being generated by the laser(s) so that the reflected light is outside of the bandwidth of the laser light being generated, the amount of interference between the reflected light and the generated working beam can be minimized.

In other embodiments, more than two optical modulators can be employed in a CRDS system to provide further extinguishing capacity to more quickly extinguish the working beam at the commencement of a ring-down event.

One or more focusing lenses can be employed in other embodiments to mode match the ring-down cavity.

Analysis of the gaseous samples can be performed at pressure levels other than one atmosphere in other embodiments. The breadth of the absorption spectrum may change accordingly.

Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages.

Claim 1:
A cavity ring-down spectroscopy system, comprising:
at least one laser (<NUM>) that generates a light beam;
a first optical modulator (<NUM>) positioned to attenuate the light beam from the at least one laser (<NUM>);
a second optical modulator (<NUM>) positioned to attenuate the light beam from the first optical modulator (<NUM>);
a ring-down cavity (<NUM>) positioned to receive the light beam from the second optical modulator (<NUM>);
a controller connected to the first optical modulator (<NUM>) and the second optical modulator (<NUM>), the controller configured to direct the first optical modulator (<NUM>) to attenuate the light beam by about <NUM>% and to simultaneously direct the second optical modulator (<NUM>) to further attenuate the light beam by about <NUM>% to reduce or extinguish an intensity of the light beam from the first optical modulator (<NUM>); and
at least one light sensor to measure an intensity of light leaked from the ring-down cavity (<NUM>).