Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-11-30
2003-04-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S481000
Reexamination Certificate
active
06545759
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical devices and, more specifically, to an optical interferometric sensor.
2. Description of the Prior Art
Sensors for detecting and measuring absolute or relative values of physical quantities such as chemical or biochemical concentration, magnetic or electric field strengths, pressure, strain, temperature, and pH, for example, in an environment to which the sensor is exposed, are well known in the art. Prior art sensors include optic sensors which provide measured values directly or by means of transducers. Interferometric type optic sensors are highly sensitive. Such sensors employ an interferometer to provide information about a condition sensed. An interferometer is an instrument that splits light from an input source into two light beams and, after the light beams are caused to travel through different paths, recombines the two beams resulting in interference and an interference pattern. An analysis of the interference pattern provides a sensitive measure of the difference in effective path length of the two optical paths.
Integrated optic sensors are monolithic structures characterized by the integration of various optical components into a single optic waveguide construction. An integrated optic sensor is typically a thin-film device comprising a waveguide constructed on a single substrate, which generally provides other optical elements or components to diffract, refract or reflect different beam portions propagating in the waveguide for purposes of separating or combining them. Integrated optic technology is particularly useful in providing the optical elements heretofore associated with interferometric sensors employing separate and discrete optical components. The prior art now includes integrated optic sensors that incorporate a variety of components including lenses, sensing fields, and filters on a single substrate.
A typical integrated optic sensor comprises one or more channel waveguides fabricated as a planar construct on a substrate. A channel waveguide is a linear structure of typically small cross-section, on the order of several micrometers wide by several micrometers high, providing an optical path for a propagating light beam. The index of refraction of the channel waveguide is higher than the index of refraction of the surrounding or supporting substrate. A light source and possibly a coupling mechanism are provided to cause a light beam to propagate within the channel waveguide. The light source can be a laser, a light emitting diode (LED), or an incandescent light source. The propagating light beam passes through a sensing region of the channel waveguide which is reactive to particular conditions of the environment. The environment may cause changes in the propagation characteristics of the channel waveguide, such as a change in the refractive index. The change in the refractive index changes the effective optical path length through the channel region, thereby changing the phase of the light beam as it emerges from the channel waveguide. Alternatively, if the channel waveguide is not directly sensitive to a particular environment, it may be coated with a material that is reactive to the environment, or to a component thereof, causing a change in the refractive index the light beam encounters. An optical output beam from the sensor can therefore be used for measuring the relative or absolute value of the condition of the environment.
The optical input beam propagates through the waveguide in modes which satisfy Maxwell's equations. Maxwell's equations govern the electric and magnetic fields of an electromagnetic wave propagating through a medium. The modes may be characterized by the frequency, polarization, transverse field distribution and phase velocity of the constituent waves. In rectangular channel waveguides the modes are designated as TE
m,n
and TM
m,n
which are orthogonally polarized components of the light beam, transverse electric and transverse magnetic, respectively, with mode number indices m and n taking non-negative integer values. Each mode represents a different field distribution corresponding to the number of wave nodes across the waveguide in each direction. The allowed modes are determined in part by the configuration of the boundaries of the waveguide, which for integrated optic sensors are the interfaces between the substrate and waveguide, the environment and the waveguide, and the coating and the waveguide. Depending on the boundaries, the waveguide materials, the waveguide dimensions and the wavelength of the input light source, no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
Commercially available integrated optic interferometers include those utilizing a Mach-Zehnder interferometric technique. This technique is characterized by single mode propagation of two light beams through two light paths, then combining the two beams to produce an optical interference pattern. Generally, a Mach-Zehnder device receives a single input light beam which is then split by a beam splitter into two beams that are directed through two different channel waveguides. Changes in the optical path length of one of the waveguides are effected when the environment causes a change in its refractive index. The beams emerging from the channel waveguides are recombined to produce a single interfering beam which is indicative of the relative or absolute change caused by exposing the device to the environment.
Integrated optic interferometers employing the Mach-Zehnder configuration provide outstanding sensitivity and can be made in small sizes. These sensors, however, suffer in that they rely on two or more single-mode channel waveguides with typical cross-sectional dimensions of 0.1 &mgr;m by 3 &mgr;m each, making fabrication difficult and costly. Most importantly, the small size of the channels make efficient light coupling difficult to achieve with Mach-Zehnder interferometers. The light coupling difficulty makes this type of interferometer all but useless for many applications.
A second type of integrated optic interferometric sensor uses a planar waveguide as the planar construct. A planar waveguide is defined by only two (parallel) boundaries, rather than the four rectangular boundaries typical of a channel waveguide. In a planar waveguide, the propagating modes are designated as TE
m
and TM
m
(transverse electric and transverse magnetic, respectively), with the mode number index m taking non-negative integer values. As in the channel waveguide, the boundaries, the waveguide materials, the waveguide dimensions and the wavelength of the input light source determine whether no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
The descriptions herein of the prior art and of the invention use the term “optic” and “light,” but it must be recognized that the techniques described are phenomena of electromagnetic radiation in general. Thus, the term “optic” and “light” herein should be read as referring to any electromagnetic radiation that meets whatever constraints are imposed by the characteristics of the various components of the sensor (such as the dimensions of the optical path) and the nature of the interaction between the sensor and properties of the environment to be sensed (such as the sensitivity of the sensor as a function of wavelength). Typically, the light will be in the visible or near-visible wavelength range.
Because prior art systems usually expose the active components of the sensor to different parts of the environment, in homogeneities in the environment can give rise to inaccurate results. For example, in a solution of a substance dissolved in a liquid one region may have a first concentration whereas an adjacent region may have a different concentration. Inaccuracies in the results of the sensor's analysis can result from the two arms of the interferometer probing regions with different concentrations. Furthermore, prior art systems provide
Font Frank G.
Lee Andrew H
Muratori Alfred H.
Nacker Wayne E.
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