Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-07-20
2004-03-16
Negash, Kinfe-Michael (Department: 2633)
Optical waveguides
With optical coupler
Plural
C385S037000, C398S087000, C398S141000, C359S337100
Reexamination Certificate
active
06707962
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical fiber communications and specifically to a method for providing signal power tilt compensation in optical transmission systems.
BACKGROUND OF THE INVENTION
Fiber communication systems are prevalent in many networks. For example, optical fibers can be used for voice networks, data networks as well as hybrid networks. (e.g., networks that carry voice, data, video and possibly other types of information). These systems are especially useful to transport information over long distances.
Fiber communication systems have a number of advantages over other choices. Due to the very high frequencies at which these systems operate, a very high bandwidth can be achieved. Attenuation is also significantly lower for optical fiber than for copper cable and optical fiber systems are not affected by external electromagnetic fields. In addition, a typical optical system will include fewer repeaters.
There is, however, a limit as to the distance a light signal may travel. To increase this distance, optical amplifiers may be used. Optical amplifiers are advantageous in that they boost the optical amplitude of a signal.
One technique used to increase the capacity of an optical system is Wavelength Division Multiplexing (WDM). In this type of system, a number of signals are transmitted concurrently, each signal having an identifiable wavelength. When a large number of wavelengths are used, the system might be referred to as a Dense Wavelength Division Multiplexing (DWDM) system.
An arrayed waveguide grating (AWG) wavelength multi/demultiplexer is a device which combines and/or splits optical signals of different wavelengths for use in WDM systems. The heart of the device, the AWG, consists of a number of arrayed channel waveguides which together act like a diffraction grating in a spectrometer. The grating has a high wavelength resolution, thus attaining narrow wavelength channel spacing such as 0.8 nm in ITU channel allocation. Moreover, the multi/demultiplexer is extremely stable against mechanical vibration and has long-term reliability because it is composed of silica-based planar lightwave circuits.
SUMMARY OF THE INVENTION
In WDM optical transmission, signal powers at different wavelengths experience different amount of losses from fibers, different gains from amplifiers, and other loss variations from components such as multiplexing/demultiplexing couplers. In one aspect, this invention proposes to use the inherent loss variation of arrayed waveguide grating (AWG) multiplexing/demultiplexing coupler to cancel the effects of the fiber loss and amplifier gain variations. In the preferred embodiment, this proposal can be implemented by designing the AWG peak channel wavelength at an offset of the center channel wavelength of the transmission band.
In a first embodiment, the present invention provides a method of transmitting an optical signal. This signal is transmitted through an optical fiber and is amplified by an optical amplifier. The signal is then provided to an arrayed waveguide grating. The arrayed waveguide grating has a wavelength-dependent optical amplitude response characterized in that the wavelength having the peak optical amplitude is different than the center channel wavelength. In some instances, this may result in a situation where the total tilt effect from the optical fiber, the optical amplifier and the arrayed waveguide grating is less than the total tilt effect from the optical fiber, the optical amplifier and an arrayed waveguide grating with a standard wavelength-dependent optical amplitude response where a signal at the center channel wavelength corresponds to the peak optical amplitude of the arrayed waveguide grating.
In another aspect, the present invention provides an arrayed waveguide grating having a wavelength dependent optical amplitude response. The amplitude response ranges from a first wavelength to a second wavelength and has a center wavelength that is about half way between the first wavelength and the second wavelength. The amplitude response further has a peak wavelength where the optical amplitude is at its highest level between the first wavelength and the second wavelength. In the preferred embodiment, the peak wavelength is different than the center wavelength.
Various aspects of the present invention provide advantages. For example, in one aspect, the present invention provides a cost-effective way for balancing out signal performance across different wavelengths suffered from fiber loss and amplifier gain variation. It reduces the requirement for pre-emphasis of signal power at the source, which is sometimes insufficient to guarantee system performance and is a complicated process. It uses the common known problem of wavelength dependent loss of AWG to its advantage to proactively compensate other system component performances. This will in turn reduce the overall system cost.
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NTT Electronics, “Arrayed-Wavelength Grating Multi/Demultiplexer,” printed from www.nel.co.jp/photo/plc/awg/index.html, Jun. 26, 2000.
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Fujitsu Network Communications, Inc.
Negash Kinfe-Michael
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