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Apparatus for analyzing, identifying, or imaging a target is configured to avoid and/or prevent nulls that may occur periodically during a terahertz sweep. Exemplary apparatus may utilize a second harmonic lock-in amplifier to generate an error signal used to adjust a DC bias of a phase modulator to

Apparatus for analyzing, identifying, or imaging a target is configured to avoid and/or prevent nulls that may occur periodically during a terahertz sweep. Exemplary apparatus may utilize a second harmonic lock-in amplifier to generate an error signal used to adjust a DC bias of a phase modulator to maintain an in-phase relationship between beams to avoid nulls in an output signal during frequency sweeping.

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1. An apparatus for analyzing, identifying, or imaging a target, the apparatus comprising: a first laser configured to provide a first output beam at a first frequency;a second laser configured to provide a second output beam at a second frequency, wherein the second frequency is different than the

1. An apparatus for analyzing, identifying, or imaging a target, the apparatus comprising: a first laser configured to provide a first output beam at a first frequency;a second laser configured to provide a second output beam at a second frequency, wherein the second frequency is different than the first frequency;a source configured to receive a first optical input beam based on a first portion of the first output beam and a first portion of the second output beam and to illuminate the target with an interrogation output beam based on the first optical input beam;a detector configured to receive a second optical input beam based on a second portion of the first output beam and a second portion of the second output beam and to generate an output signal based on the second optical input beam and electromagnetic radiation from the target resulting from the interrogation output beam;a phase modulator configured to modulate the phase of one of the second portion of the first output beam and the second portion of the second output beam using a reference signal at a low frequency;a second harmonic lock-in amplifier electrically coupled to the detector to receive the output signal and configured to generate, based on the second harmonic of the reference signal and the output signal, an error signal used to adjust a DC bias of the phase modulator to maintain an in-phase relationship between the first optical input beam and the second optical input beam to avoid nulls in the output signal during frequency sweeping;a DC driver electrically coupled to the phase modulator and configured to adjust the DC bias voltage of the phase modulator to shift the phase of one of the second portion of the first output beam and the second portion of the second output beam; andan integrator electrically coupled to the DC driver and the second harmonic lock-in amplifier, wherein the integrator is configured to receive the error signal from the second harmonic lock-in amplifier to adjust the phase modulator DC bias voltage. 2. The apparatus of claim 1, the apparatus further comprising a reference signal source configured to provide the reference signal and electrically coupled to the second harmonic lock-in amplifier. 3. The apparatus of claim 1, the apparatus further comprising a DC source electrically coupled to the source to electrically bias the source with a selected fixed DC bias. 4. The apparatus of claim 1, the apparatus further comprising a low-noise amplifier electrically coupled to the detector for amplifying the output signal and electrically coupled to the second harmonic lock-in amplifier to deliver the amplified output signal. 5. The apparatus of claim 1, the apparatus further comprising: a first beam splitter optically coupled to the first laser and configure to provide the first portion of the first beam and the second portion of the first beam;a second beam splitter optically coupled to the second laser and configured to provide the first portion of the second beam and the second portion of the second beam;a first beam combiner optically coupled to the first beam splitter and the second beam splitter and configured to combine the first portion of the first beam and the first portion of the second beam to provide the first optical input beam; anda second beam combiner optically coupled to the first beam splitter and the second beam splitter and configured to combine the second portion of the first beam and the second portion of the second beam to provide the second optical input beam. 6. The apparatus of claim 1, wherein the source comprises: a source photoconductive switch configured to generate a free-space signal based on at least the first optical input beam; andcollimating optics configured to collimate the free-space signal to provide the interrogation output beam,wherein the detector comprises:a detector photoconductive switch configured to receive the electromagnetic radiation and generate the output signal based on the second optical input beam and received electromagnetic radiation; andfocusing optics configured to collect and focus the electromagnetic radiation from the target resulting from the interrogation output beam to the detector photoconductive switch. 7. An apparatus for analyzing, identifying, or imaging a target, the apparatus comprising: a first laser configured to provide a first output beam at a first frequency;a second laser configured to provide a second output beam at a second frequency, wherein the second frequency is different than the first frequency;a source configured to receive a first optical input beam based on a first portion of the first output beam and a first portion of the second output beam and to illuminate the target with an interrogation output beam based on the first optical input beam;a detector configured to receive a second optical input beam based on a second portion of the first output beam and a second portion of the second output beam and to generate an output signal based on the second optical input beam and electromagnetic radiation from the target resulting from the interrogation output beam;a phase modulator configured to modulate the phase of one of the second portion of the first output beam and the second portion of the second output beam using a reference signal at a low frequency;a second harmonic lock-in amplifier electrically coupled to the detector to receive the output signal and configured to generate, based on the second harmonic of the reference signal and the output signal, an error signal used to adjust a DC bias of the phase modulator to maintain an in-phase relationship between the first optical input beam and the second optical input beam to avoid nulls in the output signal during frequency sweeping;a waveform shaping circuit electrically coupled for receiving the reference signal and converting the reference signal to a waveform defining slower transitions between maximum and minimum amplitudes than the reference signal; andan AC driver electrically coupled to the phase modulator and to the waveform shaping circuit for receiving the converted reference signal to generate an output voltage swing that modulates the phase modulator to produce a peak-to-peak optical phase shift of 180 degrees. 8. An apparatus for analyzing, identifying, or imaging a target, the apparatus comprising: a first laser configured to provide a first output beam at a first frequency;a second laser configured to provide a second output beam at a second frequency, wherein the second frequency is different than the first frequency;a source configured to receive a first optical input beam based on a first portion of the first output beam and a first portion of the second output beam and to illuminate the target with an interrogation output beam based on the first optical input beam;a detector configured to receive a second optical input beam based on a second portion of the first output beam and a second portion of the second output beam and to generate an output signal based on the second optical input beam and electromagnetic radiation from the target resulting from the interrogation output beam;a phase modulator configured to modulate the phase of one of the second portion of the first output beam and the second portion of the second output beam using a reference signal at a low frequency;a second harmonic lock-in amplifier electrically coupled to the detector to receive the output signal and configured to generate, based on the second harmonic of the reference signal and the output signal, an error signal used to adjust a DC bias of the phase modulator to maintain an in-phase relationship between the first optical input beam and the second optical input beam to avoid nulls in the output signal during frequency sweeping; anda first harmonic lock-in amplifier electrically coupled to the detector to receive the output signal and configured to provide a locked-in output signal based on the first harmonic of the reference signal and the output signal for use in amplitude measurement for analyzing, identify, or imaging the target. 9. An apparatus for analyzing, identifying, or imaging a target, the apparatus comprising: a first laser configured to provide a first output beam at a first frequency;a second laser configured to provide a second output beam at a second frequency, wherein the second frequency is different than the first frequency;a source configured to receive a first optical input beam based on a first portion of the first output beam and a first portion of the second output beam and to illuminate the target with an interrogation output beam based on the first optical input beam;a detector configured to receive a second optical input beam based on a second portion of the first output beam and a second portion of the second output beam and to generate an output signal based on the second optical input beam and electromagnetic radiation from the target resulting from the interrogation output beam; anda phase modulator configured such that the DC bias of the phase modulator is adjusted to modulate the phase of the second portion of the first output beam to maintain an in-phase relationship between the first optical input beam and the second optical input beam to avoid nulls in the output signal during frequency sweeping. 10. The apparatus of claim 9, wherein the apparatus further comprises a second harmonic lock-in amplifier electrically coupled to the detector to receive the output signal and configured to generate, based on the second harmonic of the reference signal and the output signal, an error signal used to adjust the DC bias of the phase modulator to maintain the in-phase relationship between the first optical input beam and the second optical input beam to avoid nulls in the output signal during frequency sweeping. 11. A method for analyzing, identifying, or imaging a target, the method comprising: providing a first output beam at a first frequency and a second output beam at a second frequency, wherein the second frequency is different than the first frequency;illuminating the target with an interrogation output beam based on a first portion of the first output beam and a first portion of the second output beam;generating an output signal based on electromagnetic radiation from the target resulting from the interrogation output beam and a second portion of the first output beam and a second portion of the second output beam, wherein the output signal is used to analyze, identify, or image the target;modulating the second portion of the second output beam by a low-frequency waveform such that a terahertz signal is swept back and forth across a fringe between the +180 degree and −180 degree points; andadjusting a DC bias of the modulation of the second portion of the second output beam to maintain an in-phase relationship to avoid nulls in the output signal during frequency sweeping. 12. The method of claim 11, the method further comprises detecting the second harmonic of the output signal to generate an error signal used to adjust a DC bias. 13. The method of claim 11, the method further comprising detecting the first harmonic of the output signal resulting from the modulation for use in amplitude measurement for analyzing, identify, or imaging the target.