Unique gas cell constructions based on a hollow-core photonic crystal fiber are used, for example, inside a fiber ring laser cavity as an intracavity gas cell. In one embodiment, two simple terminal blocks are coupled to opposite ends of the hollow-core photonic crystal fiber. Each block features a main through-bore with an optical window at one end and an optical fiber chuck fitted at the other end, while a transverse bore intersects the main bore and features a gas fitting for connection to a gas source or vacuum pump. In another embodiment, the hollow-core photonic crystal fiber is contained within an enclosure whose exterior walls are fitted with optical windows and gas ports. Inside the enclosure, fiber clamps supports the ends of the hollow-core photonic crystal fiber at positions adjacent to an in alignment with the optical windows.

A multi-passage photoacoustic device for detecting gas includes a first portion having a stable optical cavity function and having a diameter D and having a concavity with a bend radius R, the diameter D and the bend radius R being such that

[00001]$0{\left(1-\frac{D}{R}\right)}^{2}1;$

a second portion having an acoustic resonator function and having a first end having a first diameter D1 and a second end having a second diameter D2 less than the first diameter D1, the diameter of the second portion decreasing between its first and second ends; an opening for the introduction of a light beam; a gas supply system for introducing a gas to detect; an acoustic detector coupled with the second end of the second portion.

Apparatuses, methods, and systems for detecting a substance are disclosed. One system includes a light source, an optical cavity, a cavity detector, and a processor. The light source generates a beam of electro-magnetic radiation, wherein a wavelength of the beam of electro-magnetic radiation is tuned to operate at multiple wavelengths. The optical cavity receives the beam of electro-magnetic radiation, wherein the physical characteristics of the cavity define a plurality of allowed axial-plus-transverse electro-magnetic radiation modes, wherein only a subset of the allowed axial-plus-transverse electro-magnetic radiation modes are excited when the optical cavity receives the beam of electro-magnetic radiation. The cavity detector senses electro-magnetic radiation emanating from the optical cavity. The processor operates to receive information relating to the sensed electro-magnetic radiation, and detects the substance within the optical cavity based on amplitude and/or phase of the sensed electro-magnetic radiation emanating from the optical cavity.

A detection method and detection system for a trace gas, the detection method comprising: providing a resonant cavity, a gas to be measured being filled inside of a cavity body of the resonant cavity; providing detection light rays having different frequencies, the detection light rays being incident to the inside of the resonant cavity from one end of the resonant cavity in the extending direction and exiting from the other end of the resonant cavity in the extending direction so as to obtain detection light rays carrying information of a trace gas to be measured, and the cavity body of the resonant cavity having a degree of freedom of expansion and retraction in the extending direction so that the longitudinal mode frequency of the resonant cavity matches the frequencies of the incident detection light rays; and according to the detection light rays that have different frequencies and that carry information of said trace gas, acquiring the molecular saturation absorption spectrum of said trace gas, and calculating the concentration of said trace gas. The detection system comprises: a laser generating device, the resonant cavity, a photoelectric detection device, a feedback control device and a scanning control device. At room temperature, detection light rays provided by a conventional laser are used to detect the concentration of a trace gas.

A fluid analyzer includes a substrate, a quantum cascade laser formed on a surface of the substrate and including a first light-emitting surface and a second light-emitting surface facing each other in a predetermined direction parallel to the surface, a quantum cascade detector formed on the surface and including the same layer structure as the quantum cascade laser and a light incident surface facing the second light-emitting surface in the predetermined direction, and an optical element disposed on an optical path of light emitted from the first light-emitting surface across an inspection region in which a fluid to be analyzed is to be disposed and reflecting the light to feed the light back to the first light-emitting surface.

Systems and methods for detecting trace gases utilize a resonance optical cavity and one, two or more coherent light sources coupled to the cavity through one or more cavity coupling mirrors, whereby two or more optical cavities may share the same gas volume.

Systems, devices, and methods including one or more optical cavities; one or more light sources configured to emit a specified wavelength or band of wavelengths of light; and one or more photovoltaic detectors configured to receive the emitted light that has traveled over one or more path lengths, where the one or more photovoltaic detectors are configured to detect at least one of: a first trace gas species and a second trace gas species.

Systems and methods for controlling optical feedback in an optical system having a radiation source optically coupled via mode matching optics with a resonant optical cavity. The cavity includes at least two cavity mirrors, one of which is a cavity coupling mirror, and has a plurality of optical resonance cavity modes, wherein the radiation source emits a beam of continuous wave radiation and is capable of being scanned whereby a mean optical frequency of the continuous wave radiation beam is adjustable over a range of frequencies, wherein the radiation source is responsive to optical feedback radiation emerging from the cavity, and wherein the mode matching optics couples the beam of continuous wave radiation to the cavity via the cavity coupling mirror. The radiation source and the mode matching optics are aligned so that a mode fill ratio is reduced relative to a maximum mode fill ratio, wherein for the maximum mode-fill ratio the laser beam is coupled with a fundamental cavity mode.

A highly sensitive trace gas sensor based on Cavity Ring-down Spectroscopy (CRDS) makes use of a high power, multi-mode Fabry-Perot (FP) semiconductor laser with a broad wavelength range to excite a large number of cavity modes and multiple molecular transitions, thereby reducing the detector's susceptibility to vibration and making it well suited for field deployment. The laser beam is aligned on-axis to the cavity, improving the signal-noise-ratio while maintaining its vibration insensitivity. The use of a FP semiconductor laser has the added advantages of being inexpensive, compact and insensitive to vibration. The technique is demonstrated using a laser with an output power of at least 200 mW, preferably over 1.0 Watt, (=400 nm) to measure low concentrations of Nitrogen Dioxide (NO.sub.2) in zero air. For single-shot detection, 530 ppt sensitivity is demonstrated with a measurement time of 60 s which allows for sensitive measurements with high temporal resolution.

The present invention is relative to a method of ring-down spectroscopy in saturated-absorption condition, for measuring a first concentration of a gas through a measurement of the spectrum of a molecular transition of said gas.