Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-09-24
2001-08-28
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Plural
C385S037000, C359S199200
Reexamination Certificate
active
06282337
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates in general to fiber optic communication and, more particularly, to a system and method for wavelength division multiplexing and demultiplexing.
BACKGROUND OF THE INVENTION
As the information age has evolved and Internet usage has expanded, data transmission capacity has become increasingly important. At present, much of the data transmission load rests atop fiber optic networks. These fiber optic networks provide the backbone for many, if not most, networked data transmission systems.
Fiber optic networks use glass or plastic threads (i.e., fibers) to transmit data. A typical fiber optic cable consists of a bundle of these glass or plastic threads, each of which is capable of transmitting messages modulated onto light waves.
In recent years, the data transmission capacity of fiber optic cables has increased as a result of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). With WDM and DWDM systems, signals assigned to different wavelengths are combined into a single signal for transmission over a single line or medium.
In operation, a typical DWDM system may modulate each of several data streams onto a different part of the light spectrum. For example, one data stream may have an assigned wavelength of 1543 nanometers (nm) and the next data stream may have an assigned wavelength of 1543.8 nm. In most cases, the required spacing between assigned wavelengths is set by the International Telecommunication Union (ITU). Typical spacings include 0.4 nm and 0.8 nm.
The process of multiplexing has a counterpart, demultiplexing. Demultiplexing typically refers to the separation of a transmission coming through a single line or medium back into its constituent signals for further processing. Both multiplexing and demultiplexing are integral to the operation of a DWDM system.
The actual processes of multiplexing and demultiplexing within DWDM systems have conventionally involved very expensive and difficult to manufacture devices. Many, if not most, conventional DWDM devices are integrated optics devices that require a photolithography manufacturing step, which may account for a portion of the high cost associated with such devices.
Apart from being expensive to manufacture, conventional DWDM devices have operational shortcomings. For example, DWDM devices often have a high sensitivity to light polarization. Frequently referred to as polarization dependent loss (PDL), this high sensitivity to light polarization lessens the overall effectiveness and efficiency of conventional DWDM devices.
SUMMARY OF THE INVENTION
In accordance with teachings of the present invention, a system and method for wavelength division multiplexing and demultiplexing are disclosed that provide significant advantages over conventional approaches. The disclosed embodiments allow for relatively inexpensive multiplexing and demultiplexing devices that have negligible PDL.
According to one aspect of the present disclosure, a system for wavelength division multiplexing may include a fiber optic element operable to transmit a multiplexed light signal. The system may also include a light focusing device with the fiber optic element oriented to project light through the light focusing device. An additional element may be a diffraction grating having a diffraction order greater than one. The diffraction grating may be positioned in a Littrow configuration with respect to the light focusing (i.e., light striking the diffraction grating is sent back in the plane from which it arrived).
In some embodiments, the multiplexed light signal to be demultiplexed may include spectral components having assigned wavelengths. The assigned wavelengths may actually be an assigned range of wavelengths (e.g., 1543 +/−0.4 nm). Because PDL tends to become more problematic when the groove spacing, d, of a diffraction grating approaches and falls beneath three times the wavelength of light diffracting off the diffraction grating, embodiments of the present invention may include a diffraction grating with a groove spacing, d, greater than approximately three times the longest assigned wavelength (&lgr;) of the multiplexed light. In some embodiments, the value of d/&lgr; may be larger than eight. In preferred embodiments, the value of d/&lgr; may be greater than twelve.
A preferred embodiment of a fiber optic system incorporating teachings of the present invention may also include a plurality of monochromatic fiber optic elements that are operable to carry monochromatic light. These fiber optic elements may be held in position by a multi-slotted mount or V-groove array.
According to another aspect of the present invention, a method for demultiplexing multiplexed light may include projecting multiplexed light toward a light focusing device. The multiplexed light may include spectral components, each having an assigned wavelength. The projected multiplexed light may be collimated by the light focusing device. The collimated light may then be diffracted with a diffraction grating that has a groove spacing, d, greater than three times the multiplexed light's longest assigned wavelength (&lgr;). The diffraction grating may have a diffraction order greater than one and, in preferred embodiments, the diffraction order may be greater than fourteen.
When diffracting off of the diffraction grating, each light ray may have an approximately equal angle of incidence and angle of diffraction (i.e., be in autocollimation). Rays of different wavelengths (i.e., the spectral components), however, may diffract at slightly different angles and, as such, the diffraction grating may effectively separate the multiplexed light into its spectral components. Once diffracted, the spectral components may be focused with the same light focusing device and received with a respective receiving device (e.g., a signal detector or an optical fiber).
Technical advantages of the system and method include a relatively low manufacturing cost. Conventional system that require a photolithographic step in their manufacture often cost considerably more to manufacture than a system incorporating teachings of the present invention.
Other technical advantages include a decreased operational sensitivity to the polarization of incoming light and negligible PDL. Wavelength division multiplexing and demultiplexing systems that employ gratings often seek to operate within the scalar theory of gratings. The scalar theory begins to breakdown as a grating's groove spacing decreases. This breakdown manifests itself as an increased sensitivity to light polarization.
Because conventional systems employ first order diffraction gratings that have a relatively small groove spacing, many, if not all, of these systems either have high PDL's or employ expensive equipment and/or techniques to counteract the PDL problems. The disclosed system tends to avoid these problems by using diffraction orders greater than one and groove spacings larger than three times the wavelength of the diffracted light.
Other technical advantages will be apparent to those of ordinary skill in the art in view of the following specification, claims, and drawings.
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Chen Ray T.
Horwitz James W.
Baker & Botts L.L.P.
Boutsikaris Leo
Radiant Photonics, Inc.
Spyrou Cassandra
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