Methods and apparatus are employed to enhance the efficiency and applicability of Fabry-Perot interferometers. In existing devices, light introduced at a first off-axis point on the image plane is partially transmitted through the interferometer to the detector and partially reflected to a second off-axis point on the image plane. Broadly, the invention re-uses the light reflected to the second off-axis point rather than discarding it. As one example, the reflected light is guided to a third off-axis point on the image plane for retransmission back through the interferometer to a different point on the detector. A plurality of optical fibers are employed, each having one end to receive light at an off-axis point of the image plane, and another end to re-direct the light back through the etalon from a different off-axis point of the image plane. Alternatively, the reflected light is guided to a second detector, which may, in fact, form a portion of a second interferometer. As a further alternative, a mirror or other reflective element may be used to redirect the light back to the same point on the detector. In each case, the angular coherence of the light is preferably randomized prior to the subsequent detection. A multi-mode optical fiber is preferably used to carry the redirected light, which conveniently acts to randomize the angular coherence of the light during its travel.
REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. provisional patent application Ser. No. 60/115,691, filed Jan. 13, 1999, the entire contents of which are incorporated herein by reference.
An optical displacement sensor is disclosed that provides a high signal-to-noise ratio output signal without some of the disadvantages for doing so in the prior art. An embodiment of the present invention directs a light beam toward a Fabry-Perot interferometer and detects both the reflected and transmitted optical beams that result from interaction with the Fabry-Perot interferometer. Signal processing techniques are applied to signals based on both the reflected and transmitted beams, resulting in higher signal strength and/or reduced noise in the resulting output signal.
A sensor system featuring a field angle compression telescope optical system for measuring atmospheric trace gases is provided. According to an embodiment of the present invention, the telescope optical system condenses the field angle of received light with respect to a cross track plane, while leaving the field angle with respect to an along-track plane uncompressed. Such an anamorphic telescope design provides a wide field of view, while allowing information regarding the altitude or height distribution of a gas to be obtained. According to another embodiment of the present invention, the field angle of received light is compressed by magnifying the received light by a value less than 1.0 in all directions.
A correlation filter is provided having passbands at wavelengths corresponding to the absorption spectrum of an atmospheric gas of interest. In particular, the correlation filter features narrow, non-linearly spaced passbands having center wavelengths that are correlated to the non-linearly spaced absorption lines of an atmospheric gas. A correlation filter in accordance with an embodiment of the present invention includes a compensation stack having a number of thin film layers, at least some of which have an optical thickness that is not equal to an integer multiple of one-quarter of a wavelength of an absorption line of the gas of interest. The correlation filter may be provided in association with an etalon, or may comprise a number of optical cavities. In accordance with an embodiment of the present invention, a number of absorption lines associated with an atmospheric gas may be simultaneously viewed, providing a signal indicating the presence and quantity of such gas in the atmosphere having a high signal-to-noise ratio.
A first beam of light from a laser is split by a beam splitter into a reference beam and at least one second beam of light, the latter of which is directed from an optical head into an atmosphere. Light from the at least one second beam of light scattered by molecules or aerosols in the atmosphere is collected by a corresponding at least one telescope of the optical head as at least one light signal. The at least one light signal and the reference beam are simultaneously processed by different portions of a Fabry-Perot interferometer, and resulting fringe patterns are imaged onto a detector and processed by a data processor to determine at least one associated air data product.
