Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-01-07
2001-07-03
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341300
Reexamination Certificate
active
06256138
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to wavelength-dependent fiber filters and, more particularly, to a fiber filter used for optical amplification communication systems, for example, Dense Wavelength Division Multiplexing (WDM) systems.
BACKGROUND OF THE INVENTION
Pump lasers have gained widespread acceptance and have become an indispensable component for use in modem fiber amplifier communication systems. As is well known to those skilled in the art, the pump laser generates pump light which is transmitted to a fiber amplifier. The fiber amplifier absorbs the pump light energy, and uses the absorbed energy to amplify an input signal that is then transmitted over an optical line to an intended site.
One particular pump laser which is widely used is a 980 nanometer (nm) pump laser, due to its high electrical-to-optical conversion efficiency and the low noise figure of its amplifier. The pump laser module typically includes a pump laser chip, coupling optics and/or fiber grating. The coating for the laser facets and the optical elements is typically designed for 980 nm wavelengths, to substantially prevent, if not eliminate, reflection of those wavelengths.
One amplifier used in these pump lasers is an Erbium-Doped Fiber Amplifier (EDFA), which typically has a signal wavelength of between 1530 to 1610 nm. Thus, the coating used for the optical elements is often not suited for these wavelengths, resulting in a high reflection of the signal wavelengths within the pump laser module. In addition, the EDFA is often operated in very high gain conditions, such that it generates a considerable amount of amplified spontaneous emissions (ASE) in the wavelength band (1530-1610 nm). Because the coating is not suited for those wavelengths, the ASE can leak through the wavelength-division-multiplexing (WDM) device and is reflected back to the EDFA by the laser pump module. The reflected ASE power causes a degradation of the noise figure for the EDFA.
In order to overcome this problem, others have used a WDM device with a high degree of isolation from the common port to the pump module port at the signal wavelength. While such an arrangement has proven somewhat effective, a less stringent alternative would be an improvement.
Therefore, the need exists for a system which improves the return loss within the signal band of the EDFA, and which is less stringent than prior solutions.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and system for improving the return loss of an optical amplifier communication system, without the need for additional components or for a stringent arrangement of the existing components.
Briefly, the present invention includes a fiber filter in the form of an optical fiber connected between a laser pump module and an amplifier. In one preferred embodiment, the optical fiber is configured to have a predetermined loop diameter to attenuate signals in the wavelength band of the amplifier.
According to another embodiment of the invention, the fiber filter is selected to have a predetermined cutoff wavelength to improve the return signal loss, and is preferably bent into a predetermined loop diameter.
The system of the present invention in one illustrative embodiment includes: a pump laser module that is operative to generate pump light; a wavelength-division-multiplexing device connected to the pump laser module and to a signal input to multiplex the pump light and signal light; a fiber amplifier connected to the wavelength-division-multiplexing device, the amplifier being operative to absorb the pump light and to amplify the signal light; and a fiber filter interposed between the pump laser module and the wavelength-division multiplexing device, the fiber filter being operative to attenuate signals in a predetermined wavelength band corresponding with amplified spontaneous emissions created by the amplifier.
REFERENCES:
patent: 5161050 (1992-11-01), Grasso et al.
patent: 5640268 (1997-06-01), Chesnoy
patent: 5892615 (1999-04-01), Grubb et al.
patent: 5923462 (1999-07-01), van der Plaats
patent: 6038061 (2000-03-01), Sugaya
Darby & Darby
Hellner Mark
Lucent Technologies Inc
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