Optical waveguides – Accessories – Attenuator
Reexamination Certificate
2001-10-05
2003-09-23
Nguyen, Thong (Department: 2872)
Optical waveguides
Accessories
Attenuator
C385S024000, C385S032000, C385S034000, C359S283000
Reexamination Certificate
active
06625378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention generally relates to an optical device for attenuating electromagnetic radiation which propagates along a waveguide. More particularly the invention relates to an optical device for the variable attenuation of light within optical fibers.
2. The Relevant Technology
With the advances in technology in recent years, numerous attempts have been made to increase the flow of information between government agencies, private businesses and in general, from individual to individual. The need for information is ever increasing and hence many different types of transmission lines have been developed, such as optical fibers.
An optical fiber is a type of waveguide which allows the transmission of light along its length through the property of total internal reflection. The light beam or ray carries information as it propagates along the core of the optical fiber. The majority of the light beam remains within the core material since the core has a refractive index which is higher than the cladding material which surrounds the central core.
Optical fibers may be used to transmit information over both short or long distances, however, the optical properties of the optical fiber can be engineered based on the intended use. Furthermore, in optical communications, losses occur in light intensity and power along the length of the optical fiber. These losses are caused by absorption, scattering or geometric effects associated with the manufacture and use of the optical fiber. For example, light intensity is reduced by impurities contained within the optical fiber. Light is also absorbed due to splicing or connecting optical fibers and/or microscopic bending formed during fiber manufacture. Due to the variation in light intensity, the operation of the final optical components vary greatly, resulting in degradation of performance and efficiency. Optical attenuators provide a variety of useful functions to solve the above problems.
Optical attenuators perform numerous tasks associated with optical fiber communication. One function of an attenuator is to reduce the intensity of an optical signal which enters a photosensitive component. Photosensitive components are affected by variations in light intensity. Therefore, an attenuator causes the light intensity to be within the dynamic range of the photosensitive components. By using an attenuator, damage to the component is precluded. Additionally, the component does not become insensitive to small changes in the optical signal.
In other applications, attenuators serve as noise discriminators by reducing the intensity of spurious signals received by the optical device to a level below the device's response threshold. Moreover, optical attenuators are used to reduce the power of optical signals from an input fiber to an output fiber, and especially to balance optical power between several lines of an optical system. Many optical attenuators are also capable of actively attenuating an optical signal. Variable attenuators are required in some applications where different optical components require dissimilar incident optical signals, and hence variable sensitivities and saturation points. A fixed (i.e., passive) attenuation device is impractical for this purpose.
Attenuators serve to maintain the light level at a constant to compensate for component aging i.e., loss of efficiency in fiber amplifiers and reduced laser output from source, and changing absorption in optical waveguides. Variable attenuators serve to control feedback in optical amplifier control loops to maintain a constant output (e.g., as an automatic gain control element (AGC)).
Because of the variety of operations in which optical attenuators can be used, numerous types of optical attenuators have been developed. For example, U.S. Pat. No. 5,276,747 to Pan describes a liquid crystal optical attenuator device. This device uses a liquid crystal element which is manipulated to variably attenuate an incident light ray. The light travels through the liquid crystal element and is attenuated as the liquid crystal molecules are reoriented. Unfortunately, liquid crystals are temperature sensitive and hence as the temperature of the surrounding air varies, the optical properties of the liquid crystal also change. This results in inefficient transmission and inaccurate attenuation by the device.
Another group of optical attenuators are those which are mechanically activated. These vary in the method by which attenuation is caused. For example, one type of optical attenuator uses a rotatable filter placed between an input fiber and an output fiber. As the filter is rotated, the level of attenuation is varied. Another mechanical optical attenuator involves moving the ends of the input and output fibers such that the axis of the fibers are no longer aligned. By changing the angular misalignment between the input fiber and the output fiber the quantity of light transmitted along the output fiber is varied, hence variable attenuation occurs. In yet another mechanical attenuator the mechanism bends the optical fiber around a tapered or cylindrical element. As the element is rotated, different bending radii occur along the length of the optical fiber, thereby inducing radiation losses and hence variable attenuation. An alternative method of mechanically varying the attenuation of an optical signal is to use a moveable reflector, such as a mirror. As the mirror is moved, the angle at which the incident ray is reflected varies, thereby varying the light directed towards the output optical fiber, and causing attenuation.
Unfortunately, mechanical optical attenuators have the disadvantage that they use mechanical elements to either move the optical fiber or move elements which effect the optical fiber. These mechanical elements wear and eventually fail, resulting in a loss of attenuation. Furthermore, mechanical attenuators are relatively slow because of the mechanical movement compared to electrical, thermal, electromagnetic or similar movement found in other optical attenuators.
A hybrid type attenuator uses both mechanical movement of the optical fiber and modification of optical characteristics of the optical fiber. For example, U.S. Pat. No. 5,694,512 to Gonthier et al. discloses an optical attenuator which uses a tapered optical fiber. The fiber is heated and shaped to form the taper which acts to attenuate a given wavelength of light. This attenuator then may become a variable attenuator by applying a force to the optical fiber to bend the optical fiber and cause additional transmission losses. Numerous tapers and numerous bending elements may be used to vary the attenuation. Unfortunately, the movement of the optical fiber and/or bending of the optical fiber reduces the tensile strength of the optical fiber, thereby shortening the life of the fiber and hence the optical system. Furthermore, the stresses which are applied to the specific locations on the optical fiber can cause additional temperature stresses or temperature gradients along the optical fiber. The optical properties of the optical fiber are thereby changed, reducing control and effectiveness of the attenuator.
Another type of attenuator uses a series of Bragg gratings to attenuate various wavelengths of light. A Bragg grating acts as a filter to transmit and reflect different wavelengths of incident light. The Bragg grating is formed by exposing the core of the optical fiber to a beam of ultraviolet (UV) light. The UV light produces a permanent refractive index variation in the core of the optical fiber which reflects and transmits different wavelengths of incident light. Unfortunately, Bragg gratings are fixed in the optical fiber. The molecular structure of the fiber is modified to prevent the passage of a required wavelength of light. Once the optical fiber is modified, it cannot be reversed. Furthermore, one Bragg grating will only attenuate a given wavelength and cannot be varied. To form a variable attenuator using Bragg gratings it is necessary to
Adair Robert W.
Cumbo Michael J.
Hichwa Bryant P.
Mathew J. Gordon H.
O'Brien Nada A.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
JDS Uniphase Corporation
Nguyen Thong
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