Terahertz spectrometer and method for reducing photomixing interference pattern

A terahertz frequency spectrometer for performing spectroscopy on a sample in a sampling area, said spectrometer comprising:

• a first light source configured for generating a first light at a first frequency;
• a second light source for generating a second light at a second frequency, wherein the first and second lights are generated such that the first frequency and second frequency are different from one another by a difference frequency;
• a phase modulator configured for receiving a phase modulator signal at a phase modulation frequency, and for receiving at least a portion of the first light from the first light source, and for phase modulating at least a portion of the received first light relative to the second light to produce a phase modulated light that is phase modulated at the phase modulation frequency;
• a source photoconductive switch configured for receiving the phase modulated light and the second light for mixing to produce a beam of electromagnetic energy at the difference frequency in a direction toward the sample area;
• a detector photoconductive switch arranged for receiving the first light and the second light to produce a local oscillation at the difference frequency in response thereto, the detector photoconductive switch also arranged to receive at least a portion of the electromagnetic energy from the sample area as influenced by the sample and to produce a received signal that is related to the received electromagnetic energy, the detector photoconductive switch also arranged to mix the local oscillation and the received signal to produce a detector signal that is related to the difference between the local oscillation and the received signal and which includes terahertz frequency spectral information related to the sample, the detector signal including an interference pattern resulting from the detector photoconductive switch photo-mixing;
• a first harmonic detector configured to receive the detector signal and to detect a first harmonic component of the detector signal using a first harmonic of the phase modulation frequency;
• a second harmonic detector configured to receive the detector signal and to detect a second harmonic component of the detector signal using a second harmonic of the phase modulation frequency;
• and
• a controller configured to receive the detected first and second harmonic components and to combine the first and second harmonic components to produce a power curve of the terahertz frequency spectral information such that the interference pattern is at least partially eliminated.

A terahertz frequency spectrometer for performing spectroscopy on a sample in a sampling area, said spectrometer comprising:

• a first light source configured for generating a first light at a first frequency;
• a second light source for generating a second light at a second frequency, wherein the first and second lights are generated such that the first frequency and second frequency are different from one another by a difference frequency;
• a phase modulator configured for receiving a phase modulator signal at a phase modulation frequency, and receiving the first light from the first light source and phase modulating a first portion of the first light and passing a second portion of the first light that is unmodulated to produce a first phase modulated light at the phase modulation frequency, and receiving the second light from the second light source and phase modulating a first portion of the second light and passing a second portion of the second light that is unmodulated to produce a second phase modulated light at the phase modulation frequency;
• a source photoconductive switch configured for receiving the first phase modulated light and the second phase modulated light for mixing to produce a beam of electromagnetic energy at the difference frequency in a direction toward the sample area;
• a detector photoconductive switch arranged for receiving the first unmodulated light and the second unmodulated light to produce a local oscillation at the difference frequency in response thereto, the detector photoconductive switch also arranged to receive at least a portion of the electromagnetic energy from the sample area as influenced by the sample and to produce a received signal that is related to the received electromagnetic energy, the detector photoconductive switch also arranged to mix the local oscillation and the received signal to produce a detector signal that is related to the difference between the local oscillation and the received signal and which includes information related to the sample, the detector signal including an interference pattern resulting from the detector photoconductive switch photo-mixing;
• a first harmonic detector configured to receive the detector signal and to detect a first harmonic component of the detector signal using the phase modulation signal;
• a second harmonic detector configured to receive the detector signal and to detect a second harmonic component of the detector signal using the phase modulation signal;
• and
• a controller configured to receive the detected first and second harmonic components and to combine the first and second harmonic components to produce a power curve of the terahertz frequency spectral information such that the interference pattern is at least partially eliminated.