A non-relayed reflective triplet and a double-pass imaging spectrometer including the reflective triplet configured as its objective. In one example the reflective triplet includes a primary mirror that receives and reflects electromagnetic radiation from a viewed scene and defines an optical axis of the optical system, a secondary mirror that receives and reflects the electromagnetic radiation reflected from the primary mirror, and a tertiary mirror that receives the electromagnetic radiation reflected from the secondary mirror and focuses the electromagnetic radiation onto an image plane to form an image of the viewed scene. The primary, secondary, and tertiary mirrors together are configured to form a virtual exit pupil for the optical system, the image plane being located between the tertiary mirror and the virtual exit pupil. The reflective triplet is on-axis in aperture and off-axis in field of view.

An imaging spectrometer receives a beam of light from a slit and outputs the beam of light to a focal plane. The output beam of light at the focal plane is dispersed in accordance with a spectral composition of the beam of light received from the slit. The imaging spectrometer comprises first to fourth curved reflective portions. The first to fourth curved reflective portions are arranged so that the beam of light, in its passage from the slit to the focal plane, sequentially strikes the first to fourth curved reflective portions and is reflected by the first to fourth curved reflective portions. Further, the first to fourth curved reflective portions are alternatingly concave or convex, respectively, along the passage of the beam of light. At least one of the first to fourth curved reflective portions has a reflective grating structure. Further disclosed is a method of manufacturing such imaging spectrometer.

A gas-sensing apparatus is provided. The gas-sensing apparatus comprises a test chamber formed in a body and comprising a pair of micromirrors. One of the pair of micromirrors is disposed on a first surface of the body and the other of the pair of micromirrors is disposed on a second surface of the body, forming an optical cavity. A light inlet is arranged to couple light into the optical cavity, and light outlets are arranged to receive light from the optical cavity. A gas inlet configured to allow gas from outside of the detector to enter the test chamber. A gas detector comprising a gas-sensing apparatus, a light emitting system, and a light detecting system is also provided.

An imaging spectrometer receives a beam of light from a slit and outputs the beam of light to a focal plane. The output beam of light at the focal plane is dispersed in accordance with a spectral composition of the beam of light received from the slit. The imaging spectrometer comprises first to fourth curved reflective portions. The first to fourth curved reflective portions are arranged so that the beam of light, in its passage from the slit to the focal plane, sequentially strikes the first to fourth curved reflective portions and is reflected by the first to fourth curved reflective portions. Further, the first to fourth curved reflective portions are alternatingly concave or convex, respectively, along the passage of the beam of light. At least one of the first to fourth curved reflective portions has a reflective grating structure. Further disclosed is a method of manufacturing such imaging spectrometer.

An optical device includes: a diffraction grating; a depolarization plate containing a birefringent material to eliminate polarization dependency of the diffraction grating; and an optical corrector configured to optically correct diffraction angle deviation of diffracted light due to diffraction at the diffraction grating. The optical corrector may be configured to bend back the diffracted light diffracted by the diffraction grating to re-emit the light to the diffraction grating.

Embodiments of the equipment described herein uses lasers stabilized in the resonant optical cavities of Fabry-Perot Interferometers (FPIs), with at least one reference interferometer, in order to cancel the effects of atmospheric variation in the measurement of the variation of the gravitational acceleration. In addition, the radio frequency used in the modulators, or the frequency beat of the independent lasers, allows the measurement of the variation of g directly in the variable frequency, which allows high-precision measurements due to the existence of accurate clocks in counters/frequency meters. In short, the gravimeter comprises a high-finesse Fabry-Perot interferometer-based sensor with fast reading time, does not require high vacuum in the chamber thereof, can be operated in motion and can be subjected to very high pressures, without an extra encapsulation, and requires low electrical power for operation, and makes gravity measurements directly in frequency.