Patent Description:
In cavity enhanced optical absorption spectroscopy systems and methods, radiation of a laser is directed into a resonant optical cavity, and the optical intensity inside the cavity is observed. The optical frequency of the laser can be periodically scanned. If it is assumed for clarity that the laser linewidth is much smaller than the cavity resonance width, at the moment when the laser light frequency coincides with a cavity mode transmission peak the optical intensity inside the resonant optical cavity reflects total cavity loss, and the total cavity loss can be quantitatively determined provided that the incident intensity and cavity parameters are known. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption of a gas mixture present in the cavity. The lower the cavity mirror losses, or equivalently, the higher each mirror's reflectivity - the smaller the absorption of the intra-cavity gas mixture that can be detected. With very high reflectivity mirrors, the laser linewidth will become too large compared to the cavity resonance width, thus limiting achievable enhancement of the gas mixture absorption by the cavity. This can be helped by narrowing the laser linewidth using optical feedback from the cavity and a laser that is sensitive or responsive to optical feedback from the cavity. With such a feedback sensitive laser, during the scan, as the frequency of the laser light approaches the frequency of one of the cavity modes, the laser locks to that mode, i.e., the laser linewidth becomes much smaller than the resonance mode width, and that regardless whether the frequency scan range of the unlocked laser may be large, in a locked condition the optical frequency of the laser will change only within the resonance peak. As the laser frequency scan continues, the laser will lose the lock to the current cavity mode and relock to the next cavity mode that it approaches. Due to the optical feedback effect, the laser optical frequency during the scan will essentially take the number of discrete values corresponding to the peaks of the cavity mode resonances that are equidistant in optical frequency. A discrete absorption spectrum of the analyzed gas can thus be obtained by sequential coupling to the entire set of the cavity modes within the scan range, and the trace gas concentration can be derived from the absorption spectrum. This sub-family of cavity-based spectroscopy systems and methods that uses optical feedback will be referred to as optical feedback cavity enhanced absorption spectroscopy (OF CEAS).

In OF CEAS, the strength of the optical feedback from the resonance cavity to the laser should be within certain limits, otherwise it may not be possible to provide reproducible scan-to-scan mode coupling as the laser scans. Previous OF CEAS systems strive to optimize optical power coupled into an external optical cavity in a manner that enhances cavity feedback. Such coupling requires weak optical feedback to the laser such that the resonant enhancement does not last more than one Free-Spectral-Range (FSR). In known OF CEAS systems and methods, complex optical components are used to control the feedback. Such components include Faraday isolators, variable optical attenuators, or polarization rotators. One example is provided in <CIT>, which uses a settable attenuator element such as a Faraday Isolator to control the amount of light fed back towards the laser. The settable attenuator actively reduces the feedback light emerging from the cavity, and incident upon the laser, to a desirable level. This does provide a way to control the feedback but at the expense of added complexity, cost, and added sources of noise and instability. For example, adverse interference effects, temperature drifts and aging drifts may result from these components in the system. Achieving high stability and high reproducibility of the optical absorption measurements becomes a major problem in such systems.

<CIT> avoids the complications in the system design of <CIT> by specifying that one of the mirrors of the external cavity (the coupling mirror) have a low transmission level. This is a much simpler approach compared to <CIT> and avoids the disadvantages associated therein. However, the approach in <CIT> presents challenges of its own. For example, the low transmission requirement is difficult to achieve batch-to-batch, and when other components of the design change, such as the laser, the amount of transmission of the coupling mirror needs to be changed accordingly. These challenges may present difficulties in a production environment.

The thesis by Ann Bergin "Applications of optical-feedback cavity-enhanced absorption spectroscopy", University of Oxford, <NUM>, discloses an OF CEAS set-up with a V-shaped cavity, wherein the feedback is controlled by an optical isolator and a variable attenuator.

The present disclosure generally provides a method according to claim <NUM> for controlling optical feedback in an optical system. Preferred embodiments are subject matter of the dependent claims.

<FIG> illustrates a cavity enhanced absorption spectroscopy (CEAS) system <NUM>. As shown, CEAS system <NUM> includes a radiation source <NUM> that emits continuous wave coherent radiation, such as continuous wave laser light, an optical cavity <NUM> and two detectors, detector <NUM> and detector <NUM>. As shown, cavity <NUM> is a V-shaped cavity defined by cavity coupling mirror <NUM> and end mirrors <NUM> and <NUM>. It should be appreciated that the cavity could be a linear cavity with two or more mirrors, or a ring shaped cavity with three or more mirrors, or may have any other structure having three or more mirrors. An enclosure or housing (not shown) provides an air tight seal for cavity <NUM> such as to allow control of the environment within the housing and hence the cavity <NUM>. The enclosure may also house other components of system <NUM> such as the radiation source <NUM> and components between radiation source <NUM> and cavity <NUM>. One or more optical components <NUM> (collectively referred to as mode matching optics <NUM>) are configured and arranged to facilitate directing radiation from source <NUM> to the optical cavity <NUM> via cavity coupling mirror <NUM> and to ensure the mode matching of the radiation source (e.g., laser) to the cavity. As used herein below, radiation source <NUM> may be referred to as laser <NUM> or laser source <NUM> or light source <NUM>, and that radiation may also be referred to as "light", however, it should be understood that a laser is merely an example of a useful radiation source and that the radiation source may include any radiation source that is capable of emitting a beam of radiation having a desired wavelength or controllable to emit a beam of radiation over a range of desired wavelengths. Useful wavelengths include wavelengths in the visible spectrum, in the infrared spectrum, the ultraviolet spectrum and any other spectrum as may be desired.

In the system shown in <FIG>, a beam splitting element <NUM> is positioned and aligned so as to allow substantially all of the incident light <NUM> emitted or generated by light source <NUM> to impinge on cavity coupling mirror <NUM>. A small portion of the incident light beam <NUM> is directed (e.g., reflected or refracted) by element <NUM> to detector <NUM>. Cavity coupling mirror <NUM>, in this embodiment, is arranged at an angle with respect to beam <NUM>, although it could be perpendicular to beam <NUM>. A portion of incident light <NUM> enters cavity <NUM> via mirror <NUM> as intra-cavity light <NUM>. The remainder of light <NUM> is reflected away by mirror <NUM>. Depending on the frequency of incident light <NUM> and the optical length of cavity <NUM> (e.g., optical length from mirror <NUM> to mirror <NUM> to mirror <NUM>) light <NUM> circulating in the cavity may build up and resonate at one or a plurality of cavity modes defined by the optical length of the cavity. A portion of the intra-cavity light <NUM> circulating in cavity <NUM> between mirrors <NUM>, <NUM> and <NUM>, emerges or escapes via mirror <NUM> and impinges on element <NUM>. Element <NUM> allows a portion <NUM> to pass back to light source <NUM>.

In the invention, radiation source <NUM> includes a laser or other coherent light source that is sensitive or responsive to optical feedback. One useful laser is a semiconductor diode laser that is sensitive to optical feedback from light <NUM> impinging on the laser from the cavity, e.g., from coupling mirror <NUM> in the current configuration. In general, useful laser sources might include diode lasers, quantum cascade lasers and solid state lasers, any external cavity laser, etc..

Light source <NUM> is also capable of being frequency scanned, whereby a mean optical frequency of the emitted radiation beam (e.g., laser beam) is adjustable over a range of frequencies. This can be accomplished as is well known, such as, for example, by adjusting the current applied to a diode laser and/or adjusting a temperature of a laser medium. In certain aspects, the cavity <NUM> is also capable of being frequency scanned, e.g., by changing or adjusting an optical length of the cavity, whereby an optical frequency of a cavity resonance peak is adjustable over a range of frequencies. Adjustment of the optical length of the cavity can include adjusting or modulating a relative position of one or more of the cavity mirrors, adjusting a pressure of the medium within cavity <NUM> or other ways as are known to one skilled in the art.

<FIG> each show alternative configurations of CEAS system <NUM> similar to the system shown in <FIG>. In <FIG>, the detector <NUM> is positioned "behind" the radiation source <NUM> and is configured to detect light exiting the backside of source <NUM>. In <FIG>, the mode matching optics <NUM> is shown as including a single component, such as a lens element. However, it should be appreciated that the mode matching optics <NUM> may include a single component or element, e.g., lens or other optical element, or it may include multiple optical components or elements working in concert to direct the radiation beam to the cavity coupling mirror and to condition the mode matching characteristics (e.g., waist size, axis, convergence or divergence of the beam) of the radiation beam at the cavity coupling mirror so as to align the beam and cavity as desired.

CEAS system <NUM> is useful for detecting one or more trace gases within a gas mixture present in the cavity <NUM>. When the frequency of the light <NUM> emitted by source <NUM> approaches the frequency of one of the cavity modes, the light <NUM> entering the cavity <NUM> as light <NUM> begins to fill the cavity in that mode. The optical intensity of the light <NUM> circulating inside the resonance cavity reflects total cavity loss at the moment when the light frequency of light <NUM> coincides with the cavity mode transmission peak. The total cavity loss is a sum of the cavity mirror losses and losses caused by absorption by one or more components of a gas mixture present in the cavity <NUM>. Analyte absorption, e.g., absorption losses caused by absorption by the one or more gas components, is determined based on the difference of the cavity loss when the absorbing component is present, such as in an analyzed gas, and the cavity loss when the absorbing component is absent, such as in a reference gas. Absorption measurements can be made by way of direct absorption measurements or cavity ring-down measurements as are well known to those skilled in the art.

In the system shown in <FIG>, the mode-fill ratio (or mode-filling ratio) of cavity modes is typically set to an initial alignment condition that result in a maximum mode fill ratio. For a proper, optimal initial alignment of a laser beam to a cavity, the laser beam should couple to the fundamental spatial mode(s) of the cavity and should not couple to higher-order spatial modes of the cavity. An unmatched waist size between the laser beam and the cavity and/or a transverse displacement or angular displacement of the laser beam waist relative to the cavity axis and waist size may result in coupling of the beam to higher order transverse modes of the cavity, which is generally undesirable and results a reduced mode-fill ratio. As used, herein, a maximum mode-fill ratio (or mode-filling ratio) refers to the alignment conditions where the input laser beam couples to a fundamental spatial mode or modes of a cavity but not to higher order modes, and provides a maximum signal or power coupling. For example, for a Gaussian input beam, an optimum mode fill ratio may be achieved by aligning the beam axis with the cavity axis and by matching the spatial laser beam waist size with the cavity beam waist size. As used herein, a reduced mode-fill ratio (or mode-filling ratio) refers to the alignment conditions where the input laser beam couples to the fundamental spatial mode or modes of a cavity, but does not provide a maximum signal or power coupling to the cavity, and may also couple to one or more higher order modes. For example, for a Gaussian input beam, a reduced mode-fill ratio may be achieved by aligning the beam axis with the cavity axis and by reducing or increasing the spatial laser beam waist size relative to the cavity beam waist size at the cavity coupling mirror.

In the invention, the alignment of the input laser beam and the cavity is altered or adjusted so that a reduced mode-fill ratio is achieved. This may be done, for example, by altering the input beam waist size at the cavity coupling mirror (which creates a beam waist size mismatch), and/or by misaligning or tilting the axis of the input beam and the axis of the cavity slightly, and/or by creating a divergent or convergent beam (typically also with a beam waist size mismatch) at the cavity coupling mirror. Adjusting the system parameters to achieve a reduced mode fill ratio advantageously enables control of the optical feedback radiation (i.e., intensity) fed back to the feedback sensitive light source <NUM> (e.g., diode laser or other optical feedback sensitive laser or radiation source).

In a system as shown in <FIG>, the mode-matching optics <NUM> is designed so that, for an initial alignment condition, the mode size coming from the light source <NUM> matches the waist size and location of the cavity mode to provide optimum power and maximum spectral mode filling. <FIG> shows an example of initial laser-to-cavity alignment or coupling conditions wherein the input beam waste size matches the cavity beam waste size so as to provide optimum power and maximum spectral mode filling.

In an embodiment, the spacing or distance between the light source <NUM> and mode-matching optics <NUM> is adjusted to change the mode filling-ratio of the signal output by the external cavity <NUM> and detected by detector <NUM>. In certain embodiments, the distance is set after manual adjustment by a user. In another embodiment, an adjustment mechanism <NUM> is provided to automatically and controllably adjust the distance between the radiation source <NUM> and the mode matching optics <NUM> in response to a control signal. For example, in an embodiment, a relative distance between the radiation source and a component of the mode matching optics is initially aligned or set to achieve the maximum mode fill ratio, and thereafter the relative distance between the radiation source and the component of the mode matching optics is adjusted to attain the desired reduced mode fill ratio.

The adjustment mechanism <NUM> may include a mechanical actuator configured to adjust a position of the connected element or elements, such as by linearly moving the connected element(s) in a particular direction in response to a control signal. In the examples shown in <FIG>, for example, an actuator or other mechanical element may be coupled to the radiation source <NUM> and configured to move the radiation source <NUM> away from or towards the mode matching optics, or an actuator or other element may be coupled to one or more components of the mode matching optics and configured to move the connected mode matching component(s) in a linear manner (e.g., toward or away from the radiation source <NUM>), such that the desired beam waste size at the cavity coupling mirror is achieved. In certain embodiments, separate actuators may be coupled with each of the radiation source <NUM> and the one or more components of the mode matching optics to control relative positions of the connected element(s).

<FIG> show two examples of the effect of adjusting the laser-mode matching optics distance on the laser mode size and location relative to the cavity mode size and location. For example, as shown in <FIG>, the distance between the radiation source <NUM> and the mode matching optics <NUM> is decreased to produce a divergent beam of radiation interacting with the cavity mirror <NUM>. Decreasing the distance may be accomplished, as an example, by moving one or both of the radiation source <NUM> and/or a component of the mode matching optics <NUM> towards the other. Under these conditions, with a diverging radiation beam <NUM> (and increased beam waist size), the mode filling ratio is reduced relative to the initial, maximum mode-filling ratio. As shown in <FIG>, the distance between the radiation source <NUM> and the mode matching optics <NUM> is increased to produce a converging beam of radiation interacting with the cavity mirror <NUM>. Increasing the distance may be accomplished, as an example, by moving one or both of the radiation source <NUM> and/or a component of the mode matching optics <NUM> away from the other. Under these conditions, with a converging radiation beam <NUM> (and decreased beam waist size), the mode filling ratio is reduced relative to the initial, maximum mode-filling ratio.

Claim 1:
A method of controlling optical feedback in an optical system (<NUM>) having a radiation source (<NUM>) optically coupled via mode matching optics (<NUM>) with a resonant optical cavity (<NUM>) having a configuration of a V-shaped cavity having three cavity mirrors (<NUM>, <NUM>, <NUM>), one of which is a cavity coupling mirror (<NUM>), the resonant optical cavity (<NUM>) having a plurality of optical resonance cavity modes, wherein the radiation source (<NUM>) emits a beam (<NUM>) of continuous wave radiation and is capable of being scanned whereby a mean optical frequency of the beam (<NUM>) of continuous wave radiation is adjustable over a range of frequencies, wherein the radiation source (<NUM>) is responsive to optical feedback radiation emerging from the resonant optical cavity (<NUM>) and impinging on the radiation source (<NUM>), and wherein the mode matching optics (<NUM>) couples the beam (<NUM>) of continuous wave radiation to the resonant optical cavity (<NUM>) via the cavity coupling mirror (<NUM>), and wherein the cavity coupling mirror (<NUM>) is arranged at an angle with respect to the beam (<NUM>), the method comprising
aligning the radiation source (<NUM>) and the mode matching optics (<NUM>) to control an optical feedback radiation fed back from the resonant optical cavity (<NUM>) to the radiation source (<NUM>), wherein a mode fill ratio of cavity modes is reduced relative to a maximum mode fill ratio, wherein for the maximum mode-fill ratio the beam of continuous wave radiation is coupled with a fundamental cavity spatial mode, and
wherein the aligning the radiation source (<NUM>) and the mode matching optics (<NUM>) includes:
aligning or setting a relative distance between the radiation source (<NUM>) and a component of the mode matching optics (<NUM>) to achieve the maximum mode fill ratio, and thereafter
adjusting the relative distance between the radiation source (<NUM>) and the component of the mode matching optics (<NUM>) to attain the reduced mode fill ratio and control the optical feedback radiation fed back from the resonant optical cavity (<NUM>) to the radiation source (<NUM>).