Cavity enhanced laser based gas analyzer systems and methods

Inventors

Kachanov, Alexander • Koulikov, Serguei

Assignees

Li Cor Inc

Abstract

Cavity enhanced absorption spectroscopy systems and methods for detecting trace gases using a resonance optical cavity, which contains a gas mixture to be analyzed, and a laser coupled to the cavity by optical feedback. The cavity has any of a variety of configurations with two or more mirrors, including for example a linear cavity, a v-shaped cavity and a ring optical cavity. The cavity will have multiple cavity resonant modes, or a comb of frequencies spaced apart, as determined by the parameters of the cavity, including the length of the cavity, as is well known. Systems and methods herein also allow for optimization of the cavity modes excited during a scan and/or the repetition rate.

Core Innovation

The invention relates to measuring cavity loss of a resonant optical cavity over a range of frequencies by exciting one or a plurality of cavity modes in a controlled manner. The resonant optical cavity includes at least two cavity mirrors, including a cavity coupling mirror, and has a plurality of optical resonance cavity modes with frequencies within the range of frequencies of a continuous-wave laser. The laser emits continuous wave laser light, is responsive to optical feedback light emerging from the cavity, and has a mean optical frequency adjustable over a range of frequencies.

A laser drive current is applied as a series of current pulses, each having a predetermined current profile, to adjust the mean optical frequency and to excite desired cavity modes in an order during an excitation cycle determined based on the shape of the applied current pulses. An end portion of one current pulse sets the laser drive current below a laser threshold value, and the current profile of the next current pulse includes a compensation pulse portion that drives the laser at a current level and duration sufficient to compensate for some or all of the laser heat lost while the laser drive current was below the laser threshold value. The compensation pulse portion is configured to excite the next mode in the excitation order.

After a cavity mode has been excited, dynamics of intra cavity optical power are detected by measuring a free decay rate cavity ring down event and measuring a direct absorption event during the excitation cycle. A system and a gas analyzer architecture are provided with a resonant optical cavity, mode matching optics, a laser, a control module applying the pulsed current drive, and a first detector configured to measure the intra cavity optical power dynamics. A processor is included and is adapted to determine a concentration of an analyte species in the cavity responsive to a signal received from the first detector.

Claims Coverage

The patent content identifies three independent claims: a method of measuring cavity loss, a system for measuring cavity loss, and a gas analyzer for detecting analyte species with cavity loss measurements. Across these independent claims, the core inventive features repeatedly involve controlled excitation of cavity modes using a continuous-wave laser with optical feedback and a pulsed current profile that supports below-threshold operation with compensation, combined with detecting intra-cavity optical power dynamics via cavity ring-down and direct absorption events.

The independent claims collectively cover both a method and a system for measuring cavity loss across a frequency range by controlling the excitation order of cavity modes using a scanned continuous-wave laser with optical feedback and a pulsed current drive that includes below-threshold operation and a compensation pulse. They further require detecting intra-cavity optical power dynamics via free decay rate cavity ring down and direct absorption events, and in the gas analyzer claim, converting detector signals into an analyte concentration determination.

Stated Advantages

Documented Applications