Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2001-06-28
2003-05-27
Kim, Ellen E. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling function
C385S011000, C385S037000
Reexamination Certificate
active
06571034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wavelength sensitive optical components and, more specifically, to optical components utilizing a wavelength-dispersive element in combination with a static or dynamic reflective element to construct optical components such a gain flattening filters or reconfigurable, wavelength-selective routers with built-in gain flattening.
2. Background of the Invention
Communication in the form of data networks and the like increasingly relies upon optical fiber as the transmission medium of choice. Originally, fiber optic network connections were point-to-point replacements for copper wires on long links. Optical-electrical conversions were required only at each end of the optical fibers. Currently, however, all-optical network architectures are used in which optical signals are routed among different fibers of the network without needing intermediate conversions of optical signals to electrical signals and vice versa.
One popular architecture utilizes wavelength division multiplexing (WDM) in which multiple optical carriers carry data signals directed to different destinations. These carriers are impressed upon a single optical fiber. In such WDM systems, in order to avoid intermediate conversions, optical routers must be employed. In a typical, simple optical router, two input fibers carry respective sets of wavelength channels &lgr;
1
, &lgr;
2
, &lgr;
3
, . . . &lgr;
n
and &lgr;
1
′, &lgr;
2
′, &lgr;
3
′, . . . , &lgr;
n
′, respectively. The router is capable of selectively directing these different wavelength channels to different output fibers. A typical router, for instance, may direct wavelength channel &lgr;
1
to one output fiber and wavelength channel &lgr;
1
′ to another.
These routers, or similar optical components, rely on a frequency-dispersive element to accomplish their function; receiving light from one or more input waveguides (i.e., optical fibers) and dispersing it, according to frequency, to one or more output waveguides. The frequency-dispersive element typically is a dielectric filter, Bragg grating, interferometer, or free-space grating, each of these devices being well known to those skilled in the art.
All of these frequency-dispersive elements, however, have limitations. For example, dielectric filters typically exhibit stability problems and are therefore unsuitable for use in narrow-band WDM applications. Devices such as Bragg gratings and interferometers may require cascading in order to achieve high channel counts in WDM applications. In addition, Bragg gratings suffer from mechanical stability and high back reflection problems.
The frequency-dispersive element presents potential difficulties in a WDM network, particularly one in which signals may originate from many different transmitters and may travel through many routers. Each of the transmitting lasers operating at a channel wavelength &lgr;
1
(and there may be several of them originating at geographically distant points) must transmit within a given fraction of the allotted bandwidth, &dgr;&lgr;. Lasers, however, tend to drift for a number of reasons, including variation in ambient temperature and aging.
The design bandwidth &dgr;&lgr; cannot be increased without increasing the wavelength spacing &Lgr;&lgr;
SP
, hence decreasing the total number of channels (i.e., the total fiber throughput). This is because system considerations, such as amplifier bandwidths, generally limit the total wavelength span covered by all channels. Consequently, even small drifts in frequency of the laser emissions from the peak of a filter's transmission curve may cause difficulties. These frequency drifts mean that a laser signal at a filter's transmission peak may exit the router with larger amplitude than does another signal, shifted slightly down the side of the filter transmission curve. Unfortunately, these amplitude differences may be multiplied many times as the signals pass through many routers. In addition, the amplitude differences depend not only upon which laser originated the light but also upon the particular routers the signal has passed through, since filter transmission characteristics can vary from one router to another. These transmissivity differences arise from several factors such as variation of ambient temperature, aging, and differences in fabrication, which make correction difficult when the network interconnects and/or topologies change frequently.
Because of this problem, it is desirable to ensure that the frequency-dispersive element has flat band-pass transmission spectra. A flat pass-band allows laser frequencies to drift within the width of the flattened band-pass region without creating amplitude differences as described hereinabove.
A class of frequency-dispersive elements recently considered for constructing routers and similar optical devices are phased array (or “phasar”) devices. Their advantages are that they may be fabricated using conventional, well-known integrated optical circuit fabrication techniques and they support high channel counts.
One gain flattening technique using a phasar is to couple the output of single-mode waveguides into an optical interaction region containing a wavelength-dispersive element which collects light from one or more input waveguides. Light is dispersed, in accordance with its wavelength, to one or more output waveguides.
A multi-mode waveguide of a predetermined length is interposed between the optical input waveguides and the optical interaction region. This multi-mode waveguide creates a multiple-peaked image at its output in response to a single-peaked profile presented at its input end. In essence, the apparatus creates a multi-mode interference (MMI) filter having a flattened pass-band less sensitive to frequency drift of the source laser. The frequency-dispersive element is an arrayed waveguide grating disposed between a pair of optical interaction regions.
In contradistinction, the optical component of the present invention utilizes reflective arrays placed at the output side of a phasar to selectively reflect light at predetermined wavelengths. All wavelength components in a single group leave the device at essentially the same amplitude. By moving the reflective elements in relation to the phasar, the gain-flattening component may be made tunable for use in dynamic as well as static applications.
Gain flattening devices and, more particularly, gain flattening filters (GFFs) are an important class of devices that flatten the gain of fiber amplifiers. When such amplifiers are cascaded, performance, especially bandwidth, is not degraded. This is especially important in dense wavelength division multiplexing (WDM) applications where adequate and predictable gain across a relatively wide bandwidth is necessary.
Gain flattening filters have been developed using a variety of techniques for application in various wavelength ranges such as red band, blue band, etc. Technologies such as mechanical, acoustic and planar lightwave circuits, all well known to those skilled in the art, have heretofore been employed for implementing GFF devices. These prior art devices suffer from one or more limitations. Among other things, they are generally bulky and/or expensive. Also, because several optical components must sometimes be cascaded, the resulting GFFs are more subject to failure than is the simpler device of the present invention. Art relating to the claimed invention may be found in U.S. Pat. Nos. 5,412,744 to Dragone titled “Frequency Routing Device Having A Wide And Substantially Flat Passband”; 5,450,511 to Dragone titled “Efficient Reflective Multiplexer Arrangement”; 5,521,753 to Fake et al. Titled “Multi-Stage Fibre Amplifier”; and 5,881,199 to Yunn titled “Optical Branching Device Integrated With Tunable Attenuators For System Gain/Loss Equalization.”
SUMMARY OF THE INVENTION
In accordance with the present invention, there are provided planar phasar devices to implement both static and dynamic gain-flattening filters
Corning Incorporated
Douglas Walter M.
Kim Ellen E.
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