Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2000-03-06
2001-10-30
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical waveguide sensor
Reexamination Certificate
active
06310990
ABSTRACT:
BACKGROUND OF INVENTION
1. Technical Field
The present invention relates to a compression tuned optical structure; and more particularly, a compression tuned optical structure having force or displacement feedback control.
2. Description of Related Art
There are a host of applications that could exploit the principle of a tunable fiber Bragg grating. These include tunable filters, reconfigurable optical add/drop multiplexers, optical performance monitors, wavelockers, tunable lasers, etc. Each of these applications would benefit from the ability to tune the grating accurately and repeatably and without the need for optical closed loop control, i.e. without needing to measure the wavelength of the grating directly.
In the art, since the wavelength of the Bragg grating is uniquely determined by the strain and the temperature of the grating, in principle, if one could simply measure the strain and the temperature of the grating at all times, then one could always know the wavelength of the grating. In practice, this is accomplished by attaching the grating to an actuator such as a piezoelectric element, then stretching the fiber some determinable amount. If the positional relationship between the actuator and the fiber is maintained, then one can theoretically deduce the Bragg grating wavelength by measuring the displacement of the actuator.
But it is known that if there is some lost motion between the fiber and the actuator, then a measurement of the actuator displacement will result in an erroneous wavelength determination. For example, when strain tuning a coated optical fiber, this effect is almost unavoidable, as the known attachment techniques will involve some sort of epoxy with a limited holding ability. Additionally, tuning the fiber Bragg grating by applying tensile strain is considered to be an unacceptable method from the perspective of fiber reliability, since the lifetime of a fiber can be significantly reduced by continuously stressing it.
Alternatively, another known method encases the Bragg gratings in an all glass element capable of sustaining high compressional loads, which has the potential to be incorporated into a device which can be used to reliably and accurately tune a Bragg grating by strain. The technique was originally applied to pressure transducers and incorporates a glass shell around the device to enable transduction of hydrostatic pressure into compressional strain. The core of the element (the dogbone) can be used in other configurations that allow compressive loads to affect the Bragg wavelength. For example, ends of the glass element can be ground into cone shapes which fit into the cone seats of a body which is mechanically attached to a displacement actuator. This composite glass element Bragg grating has two primary advantages over standard fiber gratings discussed above from the perspective of tunability. The first is that, since the element is placed under compression rather than tension, the device is inherently more reliable. The second is that, because the device can be made of glass with arbitrary dimensions and shapes, the issue of forming a slip-free attachment to an actuator becomes simplified (e.g. glass on metal seats i.e. no epoxy to hold off high forces).
However, if one is concerned with extremely high accuracies, then one cannot ignore the possibility of lost motion or hysteresis even in the glass to metal contact region. For example, over time, the seats may deform slightly, thereby changing the actual displacement of the glass element relative to the actual displacement of the actuator. If the displacement of the actuator rather than the glass element is measured, then there will be an error introduced into the measurement.
SUMMARY OF INVENTION
The present invention provides a tunable optical device having a compression tuned optical structure and a displacement sensor.
The compression tuned optical structure responds to an optical signal, and further responds to a displacement sensor signal, for providing a compression tuned optical structure signal containing information about a change in an optical characteristic of the compression tuned optical structure, and for also further providing an excitation caused by a change in a displacement of the compression tuned optical structure.
The displacement sensor responds to the excitation, for providing the displacement sensor signal containing information about the change in the displacement of the compression tuned optical structure.
The compression tuned optical structure may be in the form of a dogbone structure that is an all-glass compression unit having wider end portions separated by a narrower intermediate portion having a Bragg grating therein.
The displacement sensor includes a capacitance sensor affixed to the compression tuned optical structure for measuring a change in capacitance between two parallel and opposing plates that depends on a change in a gap or an area with respect to the two parallel and opposing plates. The change in the displacement of the compression tuned optical structure causes a change in the gap between the two parallel and opposing plates, and the change in capacitance depends on the change in the gap. Alternatively, the change in the displacement characteristic of the compression tuned optical structure causes a change in an overlapping area between the two parallel and opposing plates, and the change in capacitance depends on the change in the overlapping area.
The capacitance sensor may have two metallic-coated tubes affixed to the compression tuned optical structure so that metallic surfaces face each other with a small gap inbetween. The two parallel and opposing plates may be affixed to parts ending from the wider end portions of the dogbone structure. The small gap may be about 200 micron. The capacitance sensor has electrodes attached to the metallic-coated tubes to allow connection of the capacitor sensor to an electronic device capable of measuring capacitance. Each of the two metallic-coated tubes is affixed to or formed on a respective one of the wider end portions. The narrower intermediate portion may have a Bragg grating or a Fabry-Perot interferometer arranged therein. The Fabry-Perot interferometer may include a pair of fiber Bragg gratings separated by a predetermined distance.
The displacement sensor may also include inductive sensing using two coils affixed to the compression tuned optical structure for measuring a change in inductance between the two coils. Other gap sensing techniques may be used, such an optical, magnetic, microwave, time-of-flight based gap sensors. Moreover, a force applied on or about the compressive element (i.e. grating or Fabry-Perot interferometer gap) may be sensed, and fed back to control the compression tuning of the optical structure.
In effect, this present invention provides a device, which combines a highly accurate means of measuring displacement with a compression tuned optical structure, including a tunable element having a fiber Bragg grating or Fabry-Perot interferometer. This hybrid device will enable a true indirect means of controlling the wavelength of the fiber Bragg grating or Fabry-Perot interferometer without the need for optical closed loop control. The device combines a highly accurate, and potentially drift-free, capacitance or inductance sensor with the tunable grating element. For example, the capacitance sensor measures displacement by taking advantage of the change in capacitance between two parallel, and opposing plates when the gap and/or the area of the plates change. Although attachment methods can be designed to minimize the creep between the actuator and the tunable glass element, in practice it is difficult to fully eliminate it. For this reason, it is highly desirable to incorporate the capacitance sensor directly onto the tunable element to form a monolithic tunable Bragg grating with built-in electronic displacement determination. Incorporating the displacement sensor directly on the glass element allows one to make a direct measurement of the displacement, whi
Bellemore David G.
Brucato Robert N.
Davis Michael A.
Helm Walter A.
Putnam Martin A.
CiDRA Corporation
Ullah Akm E.
Ware Fressola Van der Sluys & Adolphson LLP
LandOfFree
Tunable optical structure featuring feedback control does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tunable optical structure featuring feedback control, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tunable optical structure featuring feedback control will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2572220