Integrated Embedded Processor Based Laser Spectroscopic Sensor

• US 20100177316A1
• Filed: 10/06/2006
• Published: 07/15/2010
• Est. Priority Date: 09/07/2006
• Status: Active Grant

• Current Assignee: Rice University
• Original Assignee: Rice University
• Inventors: Frantz, J. Patrick, So, Stephen, Tittel, Frank K., Wysocki, Gerard
• Granted Patent: US 8,098,376 B2
• Time in Patent Office: Days
• US Class Current: 356/432
• 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 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

A novel low-power and compact laser spectroscopic sensor is described herein. Embodiments of the disclosed sensor utilize state-of-the-art microprocessors and digital processing techniques to reduce power consumption and integrate functions into a small device. In particular, novel software methods are disclosed which allow the use of low-power microprocessors which draw no more than about 0.02 W of power. Such low-power enables long battery life and allows embodiments of the sensor to be used in portable applications. In addition, the system architecture and methods described in this disclosure allow a single integrated embedded processor to control all the subsystems necessary for a laser spectroscopic sensor further reducing sensor size and power consumption. In addition, a power efficient method of calibrating a photoacoustic laser spectroscopic sensor is disclosed.

Claims

1. A laser spectroscopic sensor for detecting a compound comprising:

a detector capable of transmitting a signal in response to absorption of light by the compound;

a light source having a modulation frequency and a wavelength, wherein said light source introduces a beam of light to said detector;

a microprocessor coupled to said light source and said detector; and

software executable on said microprocessor, wherein said software causes said microprocessor to;

control the temperature, wavelength, and modulation frequency of said light source;

acquire and process data from said detector; and

generate a first waveform having a first frequency, wherein said first waveform is divisible into a plurality of different waveforms, and wherein each waveform has a frequency which is a multiple of the modulation frequency of said light source.

2. The laser spectroscopic sensor of claim 1 wherein said detector is an acoustic detector having a resonant frequency;

and

wherein said software executable on said microprocessor causes said microprocessor to iteratively generate the first waveform to tune the modulation frequency of said light source to the resonant frequency of said acoustic detector.

3. The laser spectroscopic sensor of claim 1 wherein said detector is a photodetector.

4. The laser spectroscopic sensor of claim 1, wherein said microprocessor draws no more than 0.02 W of power

5. The laser spectroscopic sensor of claim 1, further comprising a first lock-in amplifier coupled to said microprocessor and said detector, said first lock-in amplifier receives the signal from said detector and transmits an amplified signal to said microprocessor.

6. The laser spectroscopic sensor of claim 5, further comprising a plurality of frequency dividers coupled to said microprocessor.

7. The laser spectroscopic sensor of claim 6 wherein said software executable on said microprocessor further causes said microprocessor to:

a) send the first waveform to the plurality of frequency dividers to divide the first waveform in parallel into said plurality of different waveforms;

b) determine whether the amplified signal from said first lock-in amplifier has reached a maximum value; and

c) if the amplified signal has not reached a maximum value, adjust the first frequency; and

(d) repeating steps (a) through (c) until the signal has reached a maximum value so as to tune the modulation frequency of said light source to the resonant frequency of said acoustic detector.

8. The laser spectroscopic sensor of claim 6 wherein said plurality of frequency dividers comprise a plurality of asynchronous counters.

9. The laser spectroscopic sensor of claim 1 wherein the first waveform has a first frequency substantially equal to twelve times the modulation frequency of the light source.

10. The laser spectroscopic sensor of claim 1, further comprising a sample cell enclosing said detector.

11. The laser spectroscopic sensor of claim 10 wherein said software executable on said microprocessor further causes said microprocessor to control the pressure within said sample cell.

12. The laser spectroscopic sensor of claim 10 wherein the software executable on said microprocessor further causes said microprocessor to control the temperature within said sample cell.

13. The laser spectroscopic sensor of claim 7, further comprising:

a beam splitter disposed between said light source and said sample cell;

a reference cell including a photodetector, wherein said beam splitter directs a beam of light through said reference cell; and

a second lock-in amplifier coupled to said reference cell and said microprocessor, said second lock-in amplifier amplifies a signal from said reference cell for said microprocessor.

14. The laser spectroscopic sensor of claim 13 wherein said first lock-in amplifier and said second lock-in amplifier comprises an in-phase channel and a quadrature channel.

15. The laser spectroscopic sensor of claim 13 wherein said plurality of different waveforms comprises at least one phase-shifted waveform.

16. The laser spectroscopic sensor of claim 13 wherein said plurality of different waveforms comprises at least a waveform having an f frequency, a waveform having a 2f frequency, and a waveform having a 3f frequency, where f equals the modulation frequency of said light source.

17. The laser spectroscopic sensor of claim 1 wherein said software executable on said microprocessor further causes said microprocessor to adjust the wavelength of said light source to the absorption wavelength of said compound.

18. The laser spectroscopic sensor of claim 1 wherein said software executable on said microprocessor further causes said microprocessor to determine the concentration of said compound in sample cell.

19. The laser spectroscopic sensor of claim 1 wherein said software executable on said microprocessor further causes said microprocessor to perform a power conversion from a power supply to said light source.

20. The laser spectroscopic sensor of claim 1 wherein said microprocessor comprises a digital signal processor.

21. The laser spectroscopic sensor of claim 1 wherein said detector comprises a tuning fork.

22. The laser spectroscopic sensor of claim 1 wherein said detector comprises a piezoelectric material.

23. The laser spectroscopic sensor of claim 1 wherein said software uses a direct digital synthesis algorithm to generate said first waveform.

24. The laser spectroscopic sensor of claim 1 wherein said light source is a tunable diode laser.

25. The laser spectroscopic sensor of claim 1, further comprising no more than one microprocessor.

26. The laser spectroscopic sensor of claim 1, further comprising a wireless device coupled to said microprocessor.

27. The laser spectroscopic sensor of claim 1, further comprising a communications module coupled to said microprocessor, wherein said communications module is selected from the group consisting of a Universal Serial Bus Port, an IEEE 1394 High Speed Bus port, a serial port, and a parallel port.

28. The laser spectroscopic sensor of claim 1 wherein said laser spectroscopic sensor is mounted on a single circuit board.

29. A method for calibrating a laser spectroscopic sensor, wherein said laser spectroscopic sensor comprises:

an acoustic detector having a resonant frequency;

a first lock-in amplifier coupled to said acoustic detector, said first lock-in amplifier having a reference frequency; and

a light source having a modulation frequency, said method comprising;

a) generating a first waveform having a first frequency, wherein said first waveform has a first frequency greater than twice the modulation frequency of said light source;

b) forming a plurality of synchronized waveforms from the first waveform, wherein each waveform is different;

c) tuning the reference frequency of the first lock-in amplifier and the modulation frequency of the light source with the plurality of synchronized waveforms;

d) determining whether the modulation frequency of the light source is tuned to the resonant frequency of the acoustic detector; and

(e) if the modulation frequency of the light source is not tuned to the resonant frequency of the acoustic detector, adjusting the first frequency; and

f) repeating steps (a) through (e) until the modulation frequency of the light source is tuned to the resonant frequency of the acoustic detector

so as to calibrate the laser spectroscopic sensor.

30. The method of claim 29 wherein said laser spectroscopic sensor further comprises a microprocessor coupled to said lock-in amplifier and said light source.

31. The method of claim 29 wherein step (a) comprises generating the first waveform using said microprocessor.

32. The method of claim 29 wherein step (a) comprises generating a first waveform using a direct digital synthesis algorithm.

33. The method of claim 29, further comprising providing a plurality of frequency dividers coupled to said microprocessor.

34. The method of claim 33 wherein step (b) comprises forming a plurality of different waveforms by sending the first waveform to the plurality of synchronized frequency dividers, wherein the plurality of frequency divider divides the first waveform to form the plurality of synchronized waveforms.

35. The method of claim 33 wherein the plurality of synchronized frequency dividers comprise a plurality of asynchronous counters.

36. The method of claim 29 wherein the plurality of synchronized waveforms comprise at least 5 different waveforms.

37. The method of claim 29 wherein the plurality of synchronized waveforms comprise at least one waveform phase-shifted 90 degrees.

38. The method of claim 29, wherein the plurality of synchronized waveforms comprises at least a waveform having an f frequency, a waveform having a 2f frequency, and a waveform having a 3f frequency, where f equals the modulation frequency of the light source

39. The method of claim 29 wherein step (c) comprises tuning the reference frequency of the lock-in amplifier and the modulation frequency of the light source simultaneously.

40. The method of claim 30 wherein step (d) further comprises determining whether a signal from the lock-in amplifier has reached a maximum value to determine whether the modulation frequency of the light source is tuned to the resonant frequency of the acoustic detector.

41. The method of claim 29 wherein step (d) comprises determining whether a signal from the first lock-in amplifier has reached a maximum value comprises using a binary search algorithm.

42. The method of claim 29 wherein step (d) comprises determining whether a signal from the first lock-in amplifier has reached a maximum value using said microprocessor.

43. The method of claim 29 wherein step (d) comprises determining whether a signal from the first lock-in amplifier has reached a maximum value by using a lock-in amplifier to lock to the resonant frequency of the acoustic detector.