Terahertz spectrometer with phase modulation and method
First Claim
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1. A method for identifying a terahertz frequency spectral characteristic of a material in a sampling region, the method comprising:
- generating a first terahertz signal and a second terahertz signal;
modulating the phase of at least one of the first terahertz signal and second terahertz signal relative to a phase offset in a periodic manner at a modulating frequency;
transmitting a terahertz electromagnetic radiation beam, based on the first terahertz signal, to the sampling region such that a characterized terahertz radiation is produced by an interaction of the terahertz electromagnetic radiation beam with the material in the sampling region, the characterized terahertz radiation including the terahertz frequency spectral characteristic related to the material;
receiving the characterized terahertz electromagnetic radiation from the sampling region and producing a characterized terahertz signal responsive to the received radiation;
mixing the characterized terahertz signal and the second terahertz signal to produce a detector signal that includes a characterized signal component related to the characteristic of the material, and a first harmonic signal component at a first harmonic of the modulating frequency resulting from the phase modulation, and a second harmonic signal component at a second harmonic of the modulating frequency resulting, at least in part, from the phase offset which is a difference between the phases of the characterized terahertz signal and the second terahertz signal;
extracting the characterized signal component from the detector signal to identify the terahertz frequency spectral characteristic of the material in the sampling region, the extracted characterized signal component defining a characterized power curve that includes nulls which are reduced power levels at certain frequencies related to the second harmonic signal component of the detector signal; and
controlling the phase offset using the second harmonic signal component such that the nulls are, at least in part, removed from the characterized power curve.
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Abstract
A highly advantageous terahertz spectrometer with phase modulation and associated method are disclosed which utilize a second harmonic for generating an error signal to control a phase modulator to at least reduce nulls of an interference pattern.
64 Citations
17 Claims
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1. A method for identifying a terahertz frequency spectral characteristic of a material in a sampling region, the method comprising:
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generating a first terahertz signal and a second terahertz signal; modulating the phase of at least one of the first terahertz signal and second terahertz signal relative to a phase offset in a periodic manner at a modulating frequency; transmitting a terahertz electromagnetic radiation beam, based on the first terahertz signal, to the sampling region such that a characterized terahertz radiation is produced by an interaction of the terahertz electromagnetic radiation beam with the material in the sampling region, the characterized terahertz radiation including the terahertz frequency spectral characteristic related to the material; receiving the characterized terahertz electromagnetic radiation from the sampling region and producing a characterized terahertz signal responsive to the received radiation; mixing the characterized terahertz signal and the second terahertz signal to produce a detector signal that includes a characterized signal component related to the characteristic of the material, and a first harmonic signal component at a first harmonic of the modulating frequency resulting from the phase modulation, and a second harmonic signal component at a second harmonic of the modulating frequency resulting, at least in part, from the phase offset which is a difference between the phases of the characterized terahertz signal and the second terahertz signal; extracting the characterized signal component from the detector signal to identify the terahertz frequency spectral characteristic of the material in the sampling region, the extracted characterized signal component defining a characterized power curve that includes nulls which are reduced power levels at certain frequencies related to the second harmonic signal component of the detector signal; and controlling the phase offset using the second harmonic signal component such that the nulls are, at least in part, removed from the characterized power curve. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. A method for identifying a characteristic of a material in a sampling region, the method comprising:
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generating a first terahertz signal and a second terahertz signal; generating a modulating waveform having a modulating waveform frequency; modulating a phase of the first terahertz signal using the modulating waveform to produce a modulated terahertz signal; transmitting a terahertz electromagnetic beam, based on a selected one of the modulated terahertz signal and the second terahertz signal, to the sampling region such that a characterized terahertz radiation is produced by an interaction of the terahertz electromagnetic beam with the material in the sampling region, the characterized terahertz radiation including a component that is related to the characteristic of the material; receiving the characterized terahertz radiation and producing a characterized terahertz signal responsive thereto; producing a detector signal based at least in part on a difference between the characterized terahertz signal and the other one of the modulated terahertz signal and the second terahertz signal, the detector signal including a characterized signal component that is related to the characteristic of the material, and wherein the phase is modulated such that the detector signal includes a first harmonic component that is dependent, at least in part, on the modulating waveform frequency and a second harmonic component that is dependent, at least in part, on a phase offset between the characterized terahertz signal and the other one of the modulated terahertz signal and the second terahertz signal, wherein the second harmonic component is indicative of nulls in the detector signal; detecting the second harmonic component of the detector signal; controlling the phase offset during said modulation based on the detected second harmonic component such that the second harmonic component is essentially eliminated from the detector signal to thereby essentially eliminate nulls from the detector signal, and wherein the phase of the first terahertz signal is modulated relative to the phase offset; and extracting the characterized signal component from the first harmonic of the detector signal and identifying the characteristic of the material in the sampling region based on the characterized signal component.
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16. A method for identifying a terahertz frequency spectral characteristic of a material in a sampling region, the method comprising:
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generating a terahertz beam that passes through the sampling region thereby producing a characterized terahertz radiation that is influenced by the material; generating a terahertz local oscillator signal at the same frequency as the terahertz beam; applying phase modulation in a way that modulates the phase of at least one of the terahertz beam and the terahertz local oscillator signal based on a modulation input; producing an output signal based on the characterized terahertz radiation and the terahertz local oscillator signal that can exhibit a pattern of nulls resulting from the phase modulation and which includes a first harmonic that characterizes the terahertz spectral characteristic of the sample; detecting a second harmonic of the output signal to produce an error signal; and driving the modulation input based on the error signal to at least reduce the nulls. - View Dependent Claims (17)
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Specification