FIELD

The specification relates generally to resonant cavities, and, in particular, to a method for tuning a resonant cavity, and a cavity ring-down spectroscopy system.

BACKGROUND OF THE DISCLOSURE

Cavity ring-down spectroscopy (“CRDS”) is an approach that is generally used to analyze a gaseous sample via their absorption spectra. A typical CRDS system employs a laser generating a beam that is directed into a cavity of a chamber having two highly reflective mirrors. The beam is normally within the visible light spectrum, or the near infrared (“IR”) spectrum, and is tuned to a single wavelength. The beam is then reflected repeatedly between the mirrors, which allow a fraction of the light to escape the ring-down cavity.

In order to “fill” the ring-down cavity, the length of the cavity has to be in tune with the laser wavelength. This is generally done by adjusting the position of one of the two mirrors. When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference. When the light entering the cavity is extinguished, the intensity of the light in the ring-down cavity, when empty, decays at a pre-determined rate. A small fraction of the light is not reflected by the mirrors and escapes the ring-down cavity. The intensity of the escaping light is measured by a sensor component to determine the decay rate.

When the gaseous sample is placed in the ring-down cavity, analytes present in the gaseous sample absorb some of the light, thereby accelerating the decay of the intensity of the light in the ring-down cavity. Absorption spectra are generated by measuring the decay times of the light in the presence of the gaseous sample at specific wavelengths relative to the decay times of the light in the absence of the gaseous sample at these wavelengths. Identification and quantification of individual analytes in the gaseous sample can be achieved via a number of methods, such as, for example, the performance of a linear regression of the measured absorption spectra for the gaseous sample with the known absorption spectra of various analytes.

In order to “fill” the ring-down cavity through constructive interference, the length of the ring-down cavity has to be in tune with the laser wavelength. This is achieved by adjusting the cavity length by moving one of the mirrors relative to the other of the mirrors. The mirror is typically moved by one or more piezoelectric (referred to herein as “piezo”) actuators driven by a piezo driver to adjust the length of the ring-down cavity. Piezo drivers, however, produce discrete output voltages, resulting in discrete positions in which the mirror can be positioned, thus providing discrete cavity lengths between the mirrors. If none of the discrete positions and, thus, the discrete cavity lengths lead to constructive interference of the laser light of the particular wavelength, the ring-down cavity may be difficult to fill in order to perform a ring-down event. More complex/expensive piezo drivers can have more fine resolutions, but their cost can be prohibitive.

SUMMARY OF THE DISCLOSURE

In one aspect, there is provided a method of tuning a resonant cavity, comprising actuating a first mirror at a first end of a resonant cavity to move in a direction between a first position relative to a second mirror at a second end of the resonant cavity, at which a cavity length between the first mirror and the second mirror is less than a resonance length for a laser beam, and a second position relative to the second mirror, at which the cavity length is greater than the resonance length, triggering an event when the cavity length is proximal to the resonance length, and continuing to actuate the first mirror in the direction between the first position and the second position during the event.

The event can be a first event, the direction can be a first direction, and the method can further include, after the continuing, actuating the first mirror to move in a second direction opposite the first direction and towards the first position, triggering a second event when the cavity length is proximal to the resonance length, and continuing to actuate the first mirror in the second direction between the second position and the first position during the second event.

The method can further include repeatedly actuating the first mirror to move in the first direction and the second direction, triggering events when the cavity length is proximal to the resonance length, and continuing to actuate the first mirror during the events.

The method can further include applying a voltage waveform to at least one piezo actuator coupled to the first mirror to actuate the first mirror between the first position and the second position.

The waveform can be sinusoidal.

The method can further include adding a base voltage to the waveform voltage applied to the at least one piezo actuator.

The method can further include controlling the base voltage to locate a light intensity peak via a light detector coupled to the resonant cavity, the peak light intensity occurring at the resonance length.

The method can further include selecting an amplitude for the voltage waveform applied to the at least one piezo actuator that actuates the first mirror less than one wavelength of a laser beam illuminating the resonant cavity, and controlling the base voltage so that two light intensity peaks are detected during each period of the voltage waveform.

The method can further include controlling the base voltage so that adjacent light intensity peaks are spaced by one half of a period of the voltage waveform.

The method can further include triggering the events to occur when a detected light intensity achieves a threshold intensity.

The events can be ring-down events.

The light detector can be coupled to a timing circuit, the timing circuit can be coupled to one of an optical modulator and a laser to extinguish the laser beam from the laser or detune the laser for the resonant cavity.

The method can further include determining expected recurrence times for a threshold intensity, and triggering the extinguishing of the laser beam or the detuning of the laser beam for the resonant cavity at the expected recurrence times for the threshold intensity.

The method can further include detecting an intensity of light in the resonant cavity via a light detector, wherein the triggering comprises triggering when a light intensity in the resonant cavity detected by the light detector achieves a threshold intensity.

The light detector can be coupled to timing circuit, and the timing circuit can be coupled to one of an optical modulator and a laser to extinguish the laser beam from the laser or detune the laser.

In another aspect, there is provided a method of tuning a resonant cavity, comprising altering a cavity length between a first mirror at a first end of a resonant cavity and a second mirror at a second end of the resonant cavity between a first cavity length that is less than a resonance length for a laser beam, and a second cavity length that is greater than the resonance length for the laser beam; triggering an event when the cavity length is proximal to the resonance length; and continuing to alter the cavity length towards the second cavity length during the event.

In a further aspect, there is provided a method of tuning a resonant cavity, comprising: actuating a first mirror at a first end of a resonant cavity to move in a direction between a first position relative to a second mirror at a second end of the resonant cavity, at which a cavity length between the first mirror and the second mirror is less than a resonance length for a laser beam, and a second position relative to the second mirror, at which the cavity length is greater than the resonance length; triggering the extinguishing of a laser beam illuminating the resonant cavity or the detuning of the laser beam for the resonant cavity when the cavity length is proximal to the resonance length; and continuing to actuate the first mirror in the direction between the first position and the second position while a light detector registers light intensity in the resonant cavity.

The direction can be a first direction, and the method can further include, after the continuing, triggering the illumination of the laser beam or the retuning of the laser beam for the resonant cavity, actuating the first mirror to move in a second direction opposite the first direction and towards the first position, triggering the extinguishing of the laser beam or the detuning of the laser beam for the resonant cavity, and continuing to actuate the first mirror in the second direction between the second position and the first position while the light detector registers light intensity in the resonant cavity.

The method can further include applying a sinusoidal waveform to at least one piezo actuator coupled to the first mirror to actuate the first mirror between the first position and the second position.

The method can further include adding a base voltage to the waveform voltage applied to the at least one piezo actuator.

The method can further include controlling the base voltage to locate a light intensity peak via a light detector coupled to the resonant cavity, the peak light intensity occurring at the resonance length.

The method can further include selecting an amplitude for the voltage waveform applied to the at least one piezo actuator that actuates the first mirror less than one wavelength of a laser beam illuminating the resonant cavity, and controlling the base voltage so that two light intensity peaks are detected during each period of the voltage waveform.

The method can further include controlling the base voltage so that adjacent light intensity peaks are spaced by one half of a period of the voltage waveform.

The cavity length being proximal to the resonance length can be detected by a detected light intensity achieving a threshold intensity.

The light detector can be coupled to a timing circuit, and the timing circuit can be coupled to one of an optical modulator and a laser to extinguish the laser beam from the laser or detune the laser for the resonant cavity.

The method can further include determining expected recurrence times for achievement of a threshold intensity as a proxy for when the cavity length is proximal to the resonance length.

In yet another aspect, there is provided a cavity ring-down spectroscopy system, comprising: a resonant cavity having a first mirror at a first end of the resonant cavity and a second mirror at a second end of the resonant cavity; a light detector coupled to the resonant cavity to measure a light intensity therein; at least one piezo actuator coupled to the first mirror to actuate the first mirror; and a control module coupled to the at least one piezo actuator to control the at least one piezo actuator to actuate the first mirror to move in a direction between a first position relative to the second mirror, at which a cavity length between the first mirror and the second mirror is less than a resonance length for a laser beam, and a second position relative to the second mirror, at which the cavity length is greater than the resonance length, trigger the extinguishing of a laser beam illuminating the resonant cavity or the detuning of the laser beam for the resonant cavity when the cavity length is proximal to the resonance length, and continue to actuate the first mirror in the direction between the first position and the second position while the light detector registers the light intensity in the resonant cavity.

The direction can be a first direction, and the control module can trigger the illumination of the laser beam or the retuning of the laser beam for the resonant cavity, actuate the first mirror to move in a second direction opposite the first direction and towards the first position, trigger the extinguishing of the laser beam or the detuning of the laser beam for the resonant cavity, and continue to actuate the first mirror in the second direction between the second position and the first position while the light detector registers light intensity in the resonant cavity.

The control module can apply a sinusoidal waveform to at least one piezo actuator coupled to the first mirror to actuate the first mirror between the first position and the second position.

The control module can add a base voltage to the waveform voltage applied to the at least one piezo actuator.

The control module can control the base voltage to locate a light intensity peak via a light detector coupled to the resonant cavity, the peak light intensity occurring at the resonance length.

The control module can select an amplitude for the voltage waveform applied to the at least one piezo actuator that actuates the first mirror less than one wavelength of a laser beam illuminating the resonant cavity, and control the base voltage so that two light intensity peaks are detected during each period of the voltage waveform.

The control module can control the base voltage so that adjacent light intensity peaks are spaced by one half of a period of the voltage waveform.

The control module can determine that the cavity length is proximal to the resonance length when a detected light intensity achieves a threshold intensity.

The light detector can be coupled to a timing circuit, and the timing circuit can be coupled to one of an optical modulator and a laser to extinguish the laser beam from the laser or detune the laser for the resonant cavity.

Other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.

