Terahertz spectrometer with phase modulation
First Claim
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1. A terahertz frequency spectrometer for identifying a terahertz frequency spectral characteristic of a material in a sampling region, comprising:
- a terahertz source for generating a first terahertz signal and 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;
a terahertz detector for generating a second terahertz signal and for receiving the characterized terahertz electromagnetic radiation from the sampling region and producing a characterized terahertz signal responsive to the received radiation, the terahertz detector mixing the characterized terahertz signal and the second terahertz signal to produce a characterized detector signal;
a phase modulator for applying a phase modulation in a way that modulates the phase of at least one of the first terahertz signal and second terahertz signal in a periodic manner at a modulating frequency, wherein the phase modulator modulates the phase relative to a phase offset that is a difference between the phases of the characterized terahertz signal and the second terahertz signal, and wherein the characterized detector signal 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;
a characteristic detector for 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
a phase offset controller for 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.
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Citations
12 Claims
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1. A terahertz frequency spectrometer for identifying a terahertz frequency spectral characteristic of a material in a sampling region, comprising:
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a terahertz source for generating a first terahertz signal and 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; a terahertz detector for generating a second terahertz signal and for receiving the characterized terahertz electromagnetic radiation from the sampling region and producing a characterized terahertz signal responsive to the received radiation, the terahertz detector mixing the characterized terahertz signal and the second terahertz signal to produce a characterized detector signal; a phase modulator for applying a phase modulation in a way that modulates the phase of at least one of the first terahertz signal and second terahertz signal in a periodic manner at a modulating frequency, wherein the phase modulator modulates the phase relative to a phase offset that is a difference between the phases of the characterized terahertz signal and the second terahertz signal, and wherein the characterized detector signal 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; a characteristic detector for 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 a phase offset controller for 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)
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12. A terahertz frequency spectrometer for identifying a terahertz frequency spectral characteristic of a material in a sampling region, comprising:
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a terahertz beam source for generating a terahertz beam that passes through the sampling region thereby producing a characterized terahertz radiation that is influenced by the material; a local oscillator generator for generating a terahertz local oscillator signal at the same frequency as the terahertz beam; a phase modulator for applying a phase modulation in a way that modulates the phase of at least one of the terahertz beam and terahertz local oscillator signal based on a modulation input; a mixer for 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; and a lock in amplifier for detecting a second harmonic of the output signal to produce an error signal for driving the modulation input based on the error signal to at least reduce the nulls.
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Specification