Terahertz frequency domain spectrometer with phase modulation of source laser beam

• US 9,029,775 B2
• Filed: 08/02/2012
• Issued: 05/12/2015
• Est. Priority Date: 05/19/2008
• Status: Active Grant

• Current Assignee: Joseph R. Demers
• Original Assignee: Joseph R. Demers
• Inventors: Demers, Joseph R., Logan, Ronald T. Jr.
• Primary Examiner(s): Bryant, Casey
• Application Number:
  US13/565,021
• Publication Number:
  US 20120326039A1
• Time in Patent Office: 1,013 Days
• Field of Search: 250/338.07, 250/338.08
• US Class Current: 250/339.07
• 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: G01J 3/4531 Devices without moving parts
• CPC Class Codes: G01N 21/3581 using far infrared light; using Terahertz radiation
• CPC Class Codes: G01N 21/3586 by Terahertz time domain spectroscopy [THz-TDS]
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

An apparatus for analyzing, identifying or imaging an target including first and second laser beams coupled to a pair of photoconductive switches to produce CW signals in one or more bands in a range of frequencies greater than 100 GHz 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 heterodyne process to generate an electrical signal representative of some characteristics of the target. The lasers are tuned to different frequencies and a phase modulator in the path of one laser beam allows the constructive or destructive interference of the signals on the detector as the laser beams are swept in frequency to be adjusted to achieve greater resolution in one or more selected frequency bands.

Claims

1. A terahertz spectrometer comprising:

first and second lasers having tunable frequencies, said first laser to produce a first output beam and said second laser to produce a second output beam, said first output beam and said second output beam having different frequencies;

a phase modulator positioned to receive a first portion of said first output beam to controllably modulate the phase of the first output beam and producing a modulated third output beam;

a signal source coupled to the phase modulator to allow the first output beam to be swept in phase over 360 degrees;

a first optical element coupled to the modulated third output beam and to a portion of the second output beam to produce a composite output beam;

a source including a first photoconductive switch activated by the composite fourth output beam to produce CW signals in a range of frequencies from 100 GHz to over 2 THz in dependence on a frequency difference between the first and second output beams;

a radiative element coupled to the source of CW signals for causing the CW signals to be substantially simultaneously focused on or through the target;

a second optical element coupled to a portion of the first output beam and to a portion of the second output beam to produce a composite fifth output beam;

a detector for acquiring spectral information from said target and coupled to the composite fifth output beam, and generating, based on said spectral information and the composite fifth output beam, an electrical signal representative of a characteristic of the target,

wherein the signal source is operable to control the phase of the CW signals for removal of an interference pattern, resulting from constructive or destructive interference between the CW signals and the composite fifth output beam on the detector as the frequency of the CW signals is swept, in the electrical signal.

2. A spectrometer as defined in claim 1, wherein the phase modulator is a lithium niobate modulator.

3. A spectrometer as defined in claim 1, wherein the frequency of signal source is selectable to allow an operator to analyze a specified frequency band of interest.

4. A spectrometer as defined in claim 1, wherein the signal source coupled to the phase modulator in swept in phase as the first output beam is swept in frequency.

5. A spectometer as defined in claim 1, further comprising a heterodyne detection system including a lock-in amplifier coupled to the detector, wherein the signal source is coupled to the lock-in amplifier.

6. A spectrometer as defined in claim 1, wherein the first and second lasers are disposed in a first housing, and the first photoconductive switch is disposed in a second housing separate from and spaced apart from the first housing, wherein the first housing and the second housing are coupled by an optical fiber.

7. A spectrometer as defined in claim 6, wherein the detector is disposed in a third housing separate from and spaced apart from the first housing, wherein the first housing and the third housing are coupled by an optical fiber.

8. A spectrometer as defined in claim 6, further comprising a power source, keypad, and display disposed in the first housing.

9. A spectrometer as defined in claim 6, further comprising a processor disposed in the first housing for determining a characteristic of the target based upon the absorption characteristics of the target in a frequency range in the 100 GHz to over 2 THz frequency band.

10. A spectrometer as defined in claim 1, wherein the detector includes a second photoconductive switch activated by the composite fifth output beam that is offset in frequency from the composite fourth output beam.

11. A spectrometer as defined in claim 10, wherein the first and second photoconductive switches are low temperature grown GaAs photoconductive switches.

12. A spectrometer as defined in claim 1, further comprising first and second thermoelectric coolers coupled to the first and second lasers, respectively, for independently coarsely tuning each of the lasers over a wavelength range of about 5 nm in intervals or step sizes of about 0.0001 nm.

13. A spectrometer as defined in claim 1, wherein the first and second lasers are DFB or DBR lasers tuned to different frequencies.

14. A spectrometer as defined in claim 1, wherein the first and second lasers are external cavity lasers.

15. A spectrometer as defined in claim 1, wherein the first optical element is a waveguide coupler.

16. A spectrometer as defined in claim 1, wherein the first photoconductive switch is biased with a constant electrical potential.

17. A method for analyzing, identifying or imaging a target, comprising:

providing first and second lasers having first and second output beams, respectively, having different frequencies;

phase modulating the first output beam to produce a third output beam;

generating a CW radiative beam in the range of frequencies from 100 GHz to over 2 THz from the second and third output beams using a first photoconductive switch;

causing the CW radiative beam to be substantially simultaneously focused on or through the target;

combining the first output beam and the second output beam into a composite fourth output beam;

acquiring a spectral information signal from said target using a second photoconductive switch activated by said composite fourth output beam;

sweeping the frequency of the CW radiative beam over a selected range of frequencies;

generating an electrical signal representative of a characteristic of said target using the spectral information signal acquired during the sweeping of the CW radiative beam over the selected range of frequencies; and

controllably adjusting the phase modulation of the first output beam so as to modify an interference pattern over the selected range of frequencies of the CW radiative beam.