Active remote sensing using a spectral lock-in technique

• US 20040212804A1
• Filed: 04/22/2003
• Published: 10/28/2004
• Est. Priority Date: 04/22/2003
• Status: Active Grant

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Inventors: Neff, Benjamin R., Dobbs, Michael E., Pruitt, Jeff D., Gypson, Matthew L.
• Granted Patent: US 6,949,734 B2
• Time in Patent Office: Days
• US Class Current: 356/435
• CPC Class Codes: G01J 3/433 Modulation spectrometry; Derivative spectrometry
• CPC Class Codes: G01N 2021/399 Diode laser
• CPC Class Codes: G01N 21/3151 using two sources of radiation of different wavelengths G01N21/33 - G01N21/39 take precedence
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

A system for sensing a characteristic of a sample may include a tunable source configured to emit optical radiation that varies over a wavelength range at a first frequency and a reference source configured to emit optical radiation that varies in amplitude at a second frequency. A science detector may be configured to detect the optical radiation from the tunable source and the reference source after interaction with the sample and generate a science signal. A number of lock-in amplifiers may be respectively configured to generate components of the science signal that are present at the first and second frequencies. A processor may be configured to determine a characteristic of the sample based on the components of the science signal that are present at the first and second frequencies.

Claims

1. A system for sensing a sample, comprising:

a first source configured to emit first optical radiation over a range of wavelengths at a first frequency;

a second source configured to emit second optical radiation at a predetermined wavelength and that is modulated at a second frequency;

a first detector configured to detect the first and second optical radiation after interaction with the sample and generate a first detection signal;

a first lock-in amplifier configured to process the first detection signal based on the first frequency to produce a first output signal; and

a second lock-in amplifier configured to process the first detection signal based on the second frequency to produce a second output signal.

2. The system of claim 1, wherein the first source includes a tunable laser.

3. The system of claim 1, further comprising:

a coupler connected to the first source and the second source and configured to combine the first optical radiation and the second optical radiation into a combined signal; and

an optical amplifier connected to the coupler and configured to amplify the combined signal.

4. The system of claim 1, further comprising:

a second detector configured to detect the first and second optical radiation before interaction with the sample and generate a second detection signal;

a third lock-in amplifier configured to process the second detection signal based on the first frequency to produce a third output signal; and

a fourth lock-in amplifier configured to process the second detection signal based on the second frequency to produce a fourth output signal.

5. The system of claim 4, wherein the sample includes a spectral feature within the range of wavelengths, the system further comprising:

a processor configured to process the first output signal, the second output signal, the third output signal, and the fourth output signal to obtain information relating to the spectral feature.

6. The system of claim 5, wherein the information includes a concentration of the sample.

7. A method of remotely sensing a sample, comprising:

transmitting a beam of optical radiation toward the sample, the beam including wavelengths periodically varying at a first frequency and an amplitude varying at a second frequency;

detecting the beam of optical radiation after interaction with the sample to produce a detection signal;

determining a portion of the detection signal that is present at the first frequency;

determining another portion of the detection signal that is present at the second frequency; and

obtaining information about the sample based on the portion of the detection signal and the another portion of the detection signal.

8. The method of claim 7, further comprising:

generating a tuning signal that periodically varies over the wavelengths at the first frequency;

generating a reference signal that is amplitude modulated at the second frequency; and

combining the tuning signal and the reference signal into the beam of optical radiation.

9. The method of claim 8, further comprising:

amplifying the beam of optical radiation before the transmitting.

10. The method of claim 7, wherein the determining a portion of the detection signal uses a lock-in technique and an external reference signal at the first frequency, and

wherein the determining another portion of the detection signal uses the lock-in technique and another external reference signal at the second frequency.

11. The method of claim 7, wherein the information about the sample includes an area of a spectral feature of the sample.

12. A system for sensing a characteristic of a sample, comprising:

a tunable source configured to emit optical radiation that varies over a wavelength range at a first frequency;

a reference source configured to emit optical radiation that varies in amplitude at a second frequency;

a science detector configured to detect the optical radiation from the tunable source and the reference source after interaction with the sample and generate a science signal;

a plurality of lock-in amplifiers respectively configured to generate components of the science signal that are present at the first and second frequencies; and

a processor configured to determine a characteristic of the sample based on the components of the science signal that are present at the first and second frequencies.

13. The system of claim 12, further comprising:

a coupler configured to combine the optical radiation from the tunable source and the reference source into a combined signal.

14. The system of claim 12, further comprising:

an optical amplifier connected to the coupler and configured to amplify the combined signal and transmit the combined signal toward the sample.

15. The system of claim 12, further comprising:

a reference detector configured to detect the optical radiation from the tunable source and the reference source before interaction with the sample and generate a reference signal;

a second plurality of lock-in amplifiers respectively configured to generate components of the reference signal that are present at the first and second frequencies.

16. The system of claim 15, wherein the processor is further configured to determine the characteristic of the sample based on the components of the reference signal that are present at the first and second frequencies.

17. The system of claim 12, wherein the characteristic includes a concentration of the sample.

18. A method of remotely sensing a target, comprising:

generating a first beam of optical radiation that is wavelength modulated at a first frequency;

generating a second beam of optical radiation that is amplitude modulated at a second frequency;

combining the first and second beams of optical radiation for transmission to the target;

generating first and second reference signals at the first and second frequencies; and

detecting first radiation and second radiation from the target using a phase sensitive technique and the first and second reference signals.

19. The method of claim 18, wherein the first beam of radiation varies over a predetermined range of wavelengths, and

wherein the second beam of radiation has a reference wavelength that falls outside the predetermined range of wavelengths.

20. The method of claim 18, further comprising:

amplifying the first and second beams of optical radiation after the combining.

21. The method of claim 18, further comprising:

detecting third radiation and fourth radiation before interaction with the target using the phase sensitive technique and the first and second reference signals.

22. The method of claim 21, further comprising:

determining one or more characteristics of the target based on the first radiation, the second radiation, the third radiation, and the fourth radiation.

23. A system for remotely sensing a sample, comprising:

means for generating an optical signal including wavelengths and amplitudes respectively varying at different frequencies; and

means for detecting optical radiation at the different frequencies using a phase sensitive technique after the optical signal has interacted with the sample.