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Cavity Buildup Dispersion Spectrometer And Performing Cavity Buildup Dispersion Spectroscopy

Patent Number: 11,209,314


A cavity buildup dispersion spectrometer includes a shutter that modulates coherent electromagnetic radiation at a shutter frequency; and produces modulated electromagnetic radiation; a frequency shifter that frequency shifts the modulated electromagnetic radiation to a shifter frequency and produces frequency shifted radiation; a resonator that produces cavity radiation from the frequency shifted radiation and the coherent electromagnetic radiation, receives an analyte; subjects the analyte to cavity radiation, and transmits the cavity radiation as transmitted electromagnetic radiation; and a receiver that: produces a detector signal from the transmitted electromagnetic radiation, such that the detector signal includes a beat frequency that corresponds to a change in a motion of resonator that includes a change in the distance between the mirrors or a change of refractive index of the analyte in the intracavity space.

patent description

The invention constitutes an apparatus and methods for performing frequency-domain optical read-out of the instantaneous electric field of laser light transmitted through an optical resonator under heterodyne buildup (or repumping) conditions at arbitrary time delay relative to an initial cavity ring-down event. The invention enables the simultaneous measurement of optical resonator round-trip light attenuation (loss) as well as round-trip phase shifts (dispersion) in the frequency domain by isolating optical beating of the transmitted electric field on a photodetector. Whereas traditional methods require multiple measurement schemes, laser scanning, or additional local oscillator beams, our invention operates with only a single probe laser beam. By judicious control of the probe beam laser frequency and power during cavity ring-down events, we initiate cavity repumping at arbitrary delays, thus controlling the desired beat signal in transmission and enabling frequency-domain absorption and dispersion spectroscopy simultaneously without a separate optical local oscillator.

The invention solves an accuracy measurement problem in the study of the complex refractive index of materials such as pure gases, trace gases and aerosols, as well as optical coatings, atomic and molecular monolayers and condensed-phase large molecules (e.g., proteins, etc.). By performing read-out of an optical beat signal (i.e., an alternating current, or AC signal), we move away from traditional read-out at direct current (DC) and thus away from flicker noise limitation and digitizer nonlinearity biases.

Example application #1, Refractive index sensing: atomic and molecular absorption, bulk material properties, resonant and non-resonant complex susceptibilities.

Example application #2, Physical sensing: length displacement, velocity, acceleration, pressure, force, acoustics.


The invention will integrate with commercial frequency counters for seamless read-out of complex refractive index and length displacement. By removing the need for advanced software approaches to line shape fitting and laser scanning, information is condensed into a single number (the beat frequency), which can be easily understood and recorded with high precision and traceability to the SI definition of the second. Such accuracy with simplicity is highly desirable and could drive commercialization.

The apparatus could be commercialized by any one of several US companies with expertise in optical instrument commercialization and cavity ring-down spectroscopy, as many of the individual optical components which comprise our apparatus (e.g., lasers, electro-optic modulators, highly reflective curved mirrors, etc.) are, in general, well known to experts in the field.

Created August 23, 2022, Updated December 14, 2023