Terahertz Frequency Domain Spectrometer with Integrated Dual Laser Module

• US 20100277726A1
• Filed: 04/04/2008
• Published: 11/04/2010
• Est. Priority Date: 04/04/2008
• Status: Active Grant

• Current Assignee: Joseph R. Demers
• Original Assignee: Emcore Corporation
• Inventors: Demers, Joseph R., Logan, Ronald T. JR., Kasper, Bryon L.
• Granted Patent: US 7,936,453 B2
• Time in Patent Office: Days
• US Class Current: 356/326
• CPC Class Codes: G01J 3/10 Arrangements of light sources specially adapted for spectrometry or colorimetry
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 21/3581 using far infrared light; using Terahertz radiation

Abstract

An apparatus for analyzing, identifying or imaging an target including an integrated dual laser module coupled to a pair of photoconductive switches to produce cw signals in the range of frequencies from 100 GHz to over 2 THz focused on and transmitted through or reflected from the target; and a detector for acquiring spectral information from signals received from the target and using a multi-spectral homodyne process to generate an electrical signal representative of some characteristics of the target with resolution less than 250 MHz. The photoconductive switches are activated by laser beams from the dual laser module. The lasers in the module are tuned to different frequencies and have two distinct low frequency identification tones respectively that are used in conjunction with a stable optical filter element to permit precise determination of the offset frequency of the lasers.

Claims

1. A laser module comprising:

a housing;

a first laser assembly disposed in said housing including a first laser for generating a first laser beam;

a second laser assembly disposed in said housing including a second laser for generating a second laser beam;

a beam splitter disposed in said housing in the path of said first and second beams for combining said beams into third and fourth optical beams;

a second beam splitter disposed in said housing in the path of said third beam and producing a fifth and sixth; and

a first window in said housing in the path of said fifth beam for permitting said fifth beam to exit the housing.

2. A laser module as defined in claim 1, wherein said second laser assembly further comprises a focusing lens in the path of the output beam from said second laser, and an isolator in the path of the output beam emitted from said lens.

3. A laser module as defined in claim 2, wherein said first laser assembly comprises a thermoelectric cooler.

4. A laser module as defined in claim 1, further comprising:

a third beam splitter disposed in said housing and in the path of said fourth beam, and functioning to split said beam into seventh and eight beams; and

a second window in said housing in the path of said seventh beam for permitting said seventh beam to exit the housing.

5. A laser module as defined in claim 1, further comprising a photodetector module disposed in said housing, including a first photodetector in the path of said sixth beam and a second photodetector in the path of said eighth beam, for measurement of the respective power of said first and second laser beams.

7. A laser module as defined in claim 1, wherein said first and second lasers are distributed feedback lasers current modulated at slightly different frequencies.

8. A laser module as defined in claim 1, wherein said first and second beams are orthogonal to each other.

6. A laser module as defined in claim 6, further comprising a wavelength filter in the path of said sixth beam in front of said first photodetector.

9. An apparatus for analyzing, identifying or imaging a target, comprising:

an integrated laser module including first and second lasers for producing a composite output beam;

a source of CW signals in the range of frequencies from 100 GHz to over 2 THz including a first photoconductive switch activated by said composite optical beam;

directing means for causing said signals to be substantially simultaneously focused on or through said target; and

a detector for acquiring spectral information reflected from said target and coupled to said composite optical beam for generating an electrical signal representative of some characteristic of the target.

10. An apparatus as in claim 10, wherein said detector includes a second photoconductive switch activated by said composite optical beam.

11. An apparatus as defined in claim 10 further comprising a processor coupled to said detector for analyzing said electrical signal and determining a characteristic of said target based upon the absorption characteristics of said target in the 100 GHz to over 2 THz frequency band.

12. An apparatus as defined in claim 10, wherein said lasers are DFB or DBR lasers with different frequencies.

13. An apparatus as defined in claim 10, wherein each of the lasers are modulated with different low frequency identification tones.

14. An apparatus as defined in claim 12 further comprising a temperature controller coupled to each of the lasers for tuning the lasers by temperature control to achieve to resolution less than 250 MHz.

15. A apparatus as defined in claim 10, wherein said integrated laser module comprises A laser module comprising:

a housing;

a first laser assembly disposed in said housing including a first laser for generating a first laser beam;

a second laser assembly disposed in said housing including a second laser for generating a second laser beam;

a beam splitter disposed in said housing in the path of said first and second beams for combining said beams into third and fourth optical beams;

a second beam splitter disposed in said housing in the path of said third beam and producing a fifth and sixth; and

a first window in said housing in the path of said fifth beam for permitting said fifth beam to exit the housing.

16. A laser module comprising:

a first laser assembly including a first laser for generating a first laser beam;

a second laser assembly including a second laser for generating a second laser beam;

a first tone generator for applying a first low frequency tone to the first laser;

a second tone generator for applying a second low frequency tone, different from said first tone, to the second laser;

a beam combiner for combing the output of said first and second lasers;

a beam splitter assembly disposed in the path of said combined beam for splitting said beams into first and second combined optical beams;

first and second photodiodes in the path of said first and second beams respectively, and each photodiode having a respective output; and

a processor for comparing the respective outputs of said first and second photodiodes for computing the wavelength of said first and second lasers.

17. A method for determining the respective wavelengths of the frequency sources in a multi-frequency composite optical beam comprising:

applying a first low frequency tone to the first laser;

applying a second low frequency tone, different from said first tone, to the second laser;

combining the beams from said first and second lasers;

splitting said combined beam into first and second combined optical beams;

detecting the power of said first beam using a first photodiode having a first spectral response;

detecting the power of said second beam using a second photodiode having a second spectral response; and

comparing the respective outputs of said first and second photodiodes and computing the wavelengths of said first and second lasers.

18. A method as defined in claim 17, further comprising filtering at least one of said first and second beams with an optical filter with a predetermined spectral response.