Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-06-27
2003-05-13
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S031000, C385S033000, C359S199200, C359S199200
Reexamination Certificate
active
06563977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for optical multiplexing and demultiplexing, and more particularly to a compact wavelength optical multiplexer-demultiplexer device providing high spectral resolution and low polarization dependency.
2. Description of the Prior Art
The explosive growth of telecommunication and computer communications, especially in the area of the Internet, has created a dramatic in increase in the volume of worldwide data traffic which has placed an increasing demand for communication networks providing increased bandwidth. To meet this demand, fiber optic (light wave) communication systems have been developed in order to harness the enormous usable bandwidth (tens of tera-Hertz) of a single optical fiber transmission link. Because it is not possible to exploit all of the bandwidth of an optical fiber using a single high capacity channel, wavelength division-multiplexing (WDM) fiber optic systems have been developed to provide transmission of multi-carrier signals over a single optical fiber thereby channelizing the bandwidth of the fiber. In accordance with WDM technology, a plurality of superimposed concurrent signals are transmitted on a single fiber, each signal having a different wavelength. WDM technology takes advantage of the relative ease of signal manipulation in the wavelength, or optical frequency domain, as opposed to the time domain. In WDM networks, optical transmitters and receivers are tuned to transmit and receive on a specific wavelength, and many signals operating on distinct wavelengths share a single fiber.
Wavelength multiplexing devices are commonly used in fiber optic communication systems to generate a single multi-carrier signal, in response to a plurality of concurrent signals having different wavelengths received from associated sources or channels, for transmission via a single fiber. At the receiving end, wavelength demultiplexing devices are commonly used to separate the composite wavelength signal into the several original signals having different wavelengths.
Dense wavelength division multiplexing (DWDM) devices provide multiplexing and demultiplexing functions in specific wavelength ranges. Important design criteria for a DWDM device include a large number of channels, narrow channel spacing, low inter-channel cross talk, low insertion loss, low polarization dependency, compactness, environmental stability, and low manufacturing cost.
U.S. patent application Ser. No. 09/193,289, filed Nov. 17, 1998, entitled “COMPACT DOUBLE-PASS WAVELENGTH MULTIPLEXER-DEMULTIPLEXER” and U.S. patent application Ser. No. 09/362,926, filed Jul. 27, 1999, entitled “COMPACT DOUBLE-PASS WAVELENGTH MULTIPLEXER HAVING AN INCREASED NUMBER OF CHANNELS” have at least one inventor in common with the present application, and are hereby incorporated by reference. Each of these Patent Applications describes a multiplexer/demultiplexer device including: a fiber mounting assembly for aligning an array of optical fibers for transmitting optical signals; collimating and focusing optics (e.g., a lens) for collimating and focusing optical beams; a transmission grating having a diffractive element that provides diffraction of optical beams; and a reflective element such as a mirror. The fiber mounting assembly supports a plurality of close-spaced optical fibers such that ends of the fibers are disposed substantially in a common plane. The collimating and focusing optics, transmission grating, and mirror are designed to provide efficient operation in selected communication wavelength regions.
During operation of one these devices as a demultiplexer, an input optical beam including a plurality of individual wavelengths is transmitted to the device via an input one of the optical fibers, and radiated from the end of the input fiber which is located at the vicinity of a focal point of the collimating lens. Divergence of the radiated input beam depends on the numerical aperture of the input fiber. The lens has a sufficient numerical aperture to accept the diverging beam from the input fiber and substantially collimate the beam which is then passed through the diffractive element causing the wavelengths of the input beam to be diffracted and separated according to their wavelengths. The spatially separated beams are redirected by the mirror back to the diffractive element which provides further spatial separation of the individual wavelengths, thereby enhancing the total dispersion effect. The spatially separated beams are then focused by the focusing lens and received directly by a plurality of output ones of the optical fibers.
During operation of the device as a multiplexer, the beam directions are essentially reversed as compared to the beam directions during operation in the demultiplexer mode. In this embodiment, each of the optical beams provided by the fibers has a different wavelength. The wavelengths of each of the beams are collimated by the lens, and then diffracted by the diffractive element with specific angular orientations according to the specific wavelength of the individual beam. The diffracted beams are reflected by the mirror, diffracted again by the diffractive element, and eventually merged into a substantially collimated beam including all of the wavelengths. This collimated beam is then focused by the lens onto an output one of the optical fibers.
Each of the devices described in U.S. patent application Ser. No. 09/193,292 and U.S. patent application Ser. No. 09/362,926 satisfies most of the important design criteria for operation of the device as a dense wavelength division multiplexer (DWDM) device. However, these devices have certain limitations which pose a difficulty in minimizing the physical size of the device while maximizing the transmission capacity.
In order to increase the transmission capacity of a fiber communication network using a DWDM device of the type described above, it is possible to increase the number of channels by decreasing the channel wavelength spacing while maintaining the physical spacing between the optical fibers in the array. The number of channels provided by a device is proportional to the linear dispersion provided by the device. The linear dispersion of a wavelength separation device of the type described above can be expressed generally in accordance with relationship (1), below.
&dgr;
L/&dgr;&lgr;=f·
(&dgr;
L/
&dgr;&thgr;)=(
f·m
)/(
d·
COS &thgr;) (1)
Where &dgr;L/&dgr;&thgr; represents linear dispersion provided by the device, f is the focal length of the collimating and focusing element, and d is the groove spacing of the diffractive element of the grating. Note that the groove spacing d of the diffractive element is inversely proportional to the groove density of the diffractive element.
Relationship (1) suggests two obvious methods of increasing the number of channels in a DWDM device. These methods include: increasing the focal length f of the collimating and focusing element; and increasing the groove density of the diffractive element of the grating. However, these methods of increasing the number of channels in the device are complicated by the fact that modern optical networks demand smaller and smaller physical device sizes while also requiring high optical performances as further explained below.
The first method of increasing the number of channels of the DWDM device includes increasing the focal lens f of the device, thereby increasing the spectral resolution of the device and reducing the channel spectral spacing between adjacent fibers. However, a consequence of increasing the focal lens f of the device is that the over-all physical size of the device will be increased. Increasing the size of the device leads to increased costs of production, and also makes environmental responses (e.g., thermal responses, stress responses, etc.) of the mechanical and optical assembly of the device difficult to control.
The second method of increasing the number of channels includes increasing the
Chen Li
Yang Wei
Zhang Shu
Bayspec, Inc.
Bovernick Rodney
Boyce Justin
Kang Juliana K.
Oppenheimer Wolff & Donnelly LLP
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