Method and apparatus for the photo-acoustic identification and quantification of analyte species in a gaseous or liquid medium

• US 20110214479A1
• Filed: 03/02/2010
• Published: 09/08/2011
• Est. Priority Date: 03/02/2010
• Status: Active Grant

• Current Assignee: Li & S Company Limited (Yesco Holdings Co., Ltd.)
• Original Assignee: Skymoon Research & Development, LLC
• Inventors: Kachanov, Alexander, Koulikov, Serguei
• Granted Patent: US 8,327,686 B2
• Time in Patent Office: Days
• US Class Current: 73/24.02
• CPC Class Codes: G01N 2021/1704 in gases
• CPC Class Codes: G01N 2021/1708 with piezotransducers probes for investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves G01N29/24
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: H01S 5/005 Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping H01S5/026, H01S5/18388 take precedence
• CPC Class Codes: H01S 5/0064 Anti-reflection components, e.g. optical isolators
• CPC Class Codes: H01S 5/0656 Seeding, i.e. an additional light input is provided for controlling the laser modes, for example by back-reflecting light from an external optical component H01S5/14, H01S5/4062 and H01S5/4006 take precedence

Abstract

A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.

Claims

5. An apparatus for the photo-acoustic identification and quantification of one or more analyte species comprising:

a) a continuous wave laser emitting a single emission line whose emission wavelength can be tuned into coincidence with an absorption line of a gaseous analyte and which is optically matched to a closed path optical cavity having at least a first and a second partially transmissive mirror whereby the laser beam enters the cavity through the first mirror,

b) a position actuator mechanically connected to either one of the cavity mirrors whereby the cavity length can be changed by applying a voltage to the actuator'"'"'s electrical input,

c) a laser control unit that comprises the electronic and mechanical elements necessary to operate the laser and perform the modulation of the laser operating wavelength and/or the laser output power according to voltage waveforms applied to the laser control unit modulation inputs, and at least a first and second analog to digital converter,

d) a resonant photo-acoustic cell situated inside the cavity whereby the optical intracavity beam traverses the resonance element of the cell with the first analog to digital converter being connected to its output,

e) a monitor photo-detector placed into the optical beam emerging from the cavity through the second mirror with the second analog to digital converter being connected to its output,

f) a data processing and control unit with both the first and second analog to digital converters being connected to its inputs and the laser control unit being connected to its output, said data processing and control unit comprising;

i. a laser wavelength modulation waveform generator whose output is connected to the laser control unit wavelength modulation input

ii. a digital processor of the analog to digital converter signals that has in-phase and quadrature outputs of the first analog to digital converter signals at the second harmonics of the wavelength modulation waveform, and in addition it has in-phase and quadrature outputs of the second analog to digital converter signals at the first harmonics of the wavelength modulation waveform, and further it has in-phase and quadrature outputs of the second analog to digital converter signals at the second harmonics of the wavelength modulation waveform,

iii. a cavity to laser lock module with the in-phase signal of the second analog to digital converter at the first harmonics of the laser wavelength modulation waveform being connected to its input, and the position actuator electrical input being connected to its output

iv. an analyte absorption module with the in-phase and quadrature outputs of the first analog to digital converter signals at the second harmonics of the wavelength modulation waveform, and the in-phase and quadrature outputs of the second analog to digital converter signals at the second harmonics of the wavelength modulation waveform being connected to its input, and the ratio of the magnitude of the first analog to digital converter signal to the magnitude of the second analog to digital converter signal directed to its output.

1. A method for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium utilizing a laser and a resonant optical cavity containing said medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal, comprising the steps of:

a) selecting the cavity parameters to cause the cavity resonance width to be equal to or larger than the frequency bandwidth of a laser beam directed into said optical cavity,

b) converting the spatial distribution of the beam to match the spatial distribution of the fundamental TEM₀₀ mode of the cavity and directing said converted beam into the cavity through said coupling cavity mirror,

c) positioning a first photo-detector in the path of the beam emerging from a cavity mirror other than the coupling mirror and connecting said first photo-detector to a DPCU,

d) applying a periodic dither waveform to the frequency of the laser beam with the initial peak to peak optical frequency deviation being equal to said cavity resonance width, if there is no stored value for this deviation, or equal to the stored value

e) moving the moveably mounted cavity mirror until the output signal of the first photo-detector in the form of a sequence of peaks and valleys thereby bringing the laser dither frequency midpoint close to the cavity resonance peak,

f) obtaining the magnitude of the in-phase and quadrature components of the first photo-detector output signal at the dither frequency by multiplying said first photo-detector output signal by a sine and a cosine reference function having the same frequency as the dither frequency and a constant phase relative to the phase of the dither waveform,

g) locking the cavity resonance to the laser with a feedback loop by directing to the movable cavity mirror assembly an electrical input signal which is proportional to said in-phase component and has a polarity that provides negative feedback,

h) adjusting the phase of the sine and cosine reference function to minimize the value of the quadrature component and storing the found phase value in order to utilize said phase value as the constant phase in subsequent operating cycles,

i) optimizing the dither amplitude by repeatedly

obtaining the magnitude of the in-phase and quadrature components of the first photo-detector output signal at the second harmonic of the dither frequency

determining the phase of the second harmonic sine and cosine reference functions that zeroes out the quadrature signal, and setting said constant phase relative to the phase of the dither waveform to this determined phase value,

determining the optimum laser wavelength dither amplitude that results in the maximum value of the in-phase component of the first photo-detector time-varying signal at the second harmonic of the laser wavelength dither frequency

j) storing the value of the optimum dither amplitude and the maximum magnitude of the in-phase component of the second harmonic of the time varying first photo-detector output signal in order to utilize said optimum dither amplitude in step d) for subsequent operating cycles,

k) periodically determining the magnitude of the in-phase and quadrature components of the photo-acoustic signal from a photo-acoustic transducer situated inside the cavity at the second harmonic of the dither frequency,

l) calculating the ratio of the photo-acoustic signal to the magnitude of the in-phase component of the first photo-detector signal at the second harmonic of the dither frequency to thereby determine a normalized photo-acoustic signal proportional to the optical absorption of said medium,

m) determining the optical absorption of said medium at the laser signal wavelength by multiplying said normalized photo-acoustic signal by a pre-determined calibration coefficient

n) incrementally scanning the laser operating wavelength and for each wavelength repeating steps f) through m),

o) maintaining the position of the movable cavity mirror within its displacement range, and if the mirror'"'"'s position approaches either of its range limits, resetting the mirror'"'"'s position back by one cavity free spectral range,

p) ensuring that the cavity maintains a locked position relative to the laser by measuring the magnitude of the in-phase component of the second harmonic of the time varying first photo-detector signal, comparing it with its value obtained in step i) and stored in step j) and if the ratio of these two values decreases below a pre-set threshold re-establishing the lock by repeating step e).

2. The method of claim 1 wherein an optical isolator situated between the laser and the cavity coupling mirror is partially transmitting whereby a portion of the optical power of the laser beam emerging from the cavity coupling mirror reenters the laser cavity, and wherein an optical element varying the optical path difference between the laser and the cavity coupling mirror is also present, and further comprising at a point during the sequence of steps f) through m) the additional steps of:

i) periodically verifying the magnitude of the quadrature component of the time varying first photo-detector output signal at the dither frequency, and

ii) increasing or decreasing the optical path difference between the laser and the cavity coupling mirror by inputting to the optical element which varies the optical path difference between the laser and the cavity coupling mirror the value proportional to the magnitude of the quadrature component of the time varying first photo-detector output signal at the dither frequency until the magnitude of the quadrature component zeroes down.

3. The method of claim 1 further comprising identifying and quantifying the analyte species when the laser wavelength scan in the pre-set range has been completed, and the set of values of the optical absorption versus the laser operating wavelength has been obtained, based on said set of values.

4. The method of claim 3 further comprising continuing the scans of the pre-set laser wavelength range and determining the time dependence of the concentration of the analyte species.

6. An apparatus in accordance with claim 5, where the laser modulation waveform generator is a direct digital synthesizer that generates an arbitrary preloaded wavelength modulation waveform from a single reference clock, which preloaded wavelength modulation waveform is non-sinusoidal.

7. An apparatus in accordance with claim 5, where the sampling clock of both the first analog to digital converter and the second analog to digital converter is the same, and is coherent with the direct digital synthesizer clock such that the direct digital synthesizer clock is an integer multiple of the both analog to digital converter sample clocks.

8. An apparatus in accordance with claim 5, where the reflectivities of each of the mirrors are chosen such that the cavity bandwidth is at least three times the laser linewidth.

9. An apparatus in accordance with claim 5, where the second mirror has a higher reflectivity than the first mirror.

10. An apparatus in accordance with claim 5 where the optical isolator is partially transmitting, and further comprising an optical element for varying the optical path difference between the laser and the first cavity mirror.

11. An apparatus in accordance with claim 5 wherein the photo-acoustic transducer is a quartz tuning fork.

12. An apparatus in accordance with claim 5 wherein the photo-acoustic transducer is a combination of a quartz tuning fork and a micro-resonator.

13. An apparatus in accordance with claim 5 wherein the photo-acoustic transducer comprises a package comprising at least two quartz tuning forks (QTF) electrically connected in parallel, and mounted next to each other such that the surface of every adjacent QTF is parallel to the immediately preceding QTF in the package, and the distance between any two adjacent QTFs is smaller than thickness of the boundary layer for the surrounding gas.

14. An apparatus in accordance with claim 13 wherein all adjacent QTFs are in direct mechanical contact with each other.

6-1. An apparatus in accordance with claim 5, where an optical isolator is situated in the laser beam path in front of said first mirror.