The present invention relates to an intrinsically gain flattened Erbium Doped fiber amplifier flattening the gain of an amplifier doped with erbium ion.
More specifically the subject invention relates to:
• a novel intrinsically gain-flattened three layers staircase profile c-band erbium doped silica based optical fiber amplifier (EDFA), having high optical gain and wide bandwidth;
The invention is a highly efficient broad-band optical fiber amplifier which fully exploit the low loss band of silica fibers and greatly contributes to wavelength division multiplexed (WDM) technologies which contributed to dramatic change in the information carrying capacity (or transmissions capacity), to bear the explosively increased telecommunication traffic, of an optical fiber link.
BACKGROUND OF THE INVENTION
Various processes for gain flattening have been reported including A-l/P- codoped EDFA, fluoride or Telluride-based EDFAs and EDFAs with low population inversion operating at 1550 nm, as compared to silica host fluoride based EDFA yield better flat gain and bandwidth.
A disadvantage of a single wavelength amplification at 1550 nm with silica based conventional EDFA is poor gain flatness over the amplification bond. This gain non-uniformity is determinate in WDM applications.
In the case of multiple stage amplification, the useful spectral range for the system is drastically reduced. Correction of gain non-uniformity by equalisation requires internal or external filtering which increase the complexity of such system.
Co-doping of germano-silica fibers with alumina broadens the gain spectrum but a large peak near 1530 nm remains.
One solution consists in using flurozirconate glass hosts like ZBLAN instead of silica, but their

splicing with silica based conventional transmission fiber results in significant coupling losses.
The use of twin core fiber in EDFAs has also been suggested in many schemes of flattening the gain by using in different techniques wherein it is difficult to have simultaneous access of the two cores while in other systems a high noise figure is obtained.
To overcome the drawbacks of the previous techniques and existing methods a novel intrinsically gain flattened - design has been developed in the subject invention of optical fibers by introducing the staircase fiber profile which is capable of providing flat gain over a wave-length span of approximately 34 nm. The subject design is fully compatible with the present conventional optical fiber link in terms of splicing and connection.
The subject invention has a useful application in broad-board WDM optical communication system because it has high optical gain, wide-board width, better pump efficiency, flat gain, uniform amplification with reduced gain excursion and good tolerance of various parameters like core radii, refractive index etc.
The subject invention is useful in the realization of gain flattened EDFAs without employing the number of extra external gain flattening components such as filters or use other mechanism like losing, thereby resulting in improved reliability of communication system.
In the subject invention gain is less for wavelengths below 1540 nm as compared to a conventional EDFA while the gain is high for wavelengths greater than 1540 nm. This results in flattering of the gain spectrum.
Changes in various parameters (case radii, refractive index, etc.) doesn't effect the phase matching condition hence gain spectrum.
In the subject invention functional power increases from the inner core to the outer core with increase in wave length. This result in better pump efficiency.
In the subject invention the noise figure is shown to decrease from about 6.5 dB at 1525 nm
to 4.5 dm around 1560 mm.
In the subject invention coupling efficiency is seen to be more than 95% of conventional undoped transmission fiber.
The subject invention of staircase profile has three layers while cronial profile has four layers. Hence it is easier to fabricate a staircase fiber than the co-axial fiber.
The subject invention is shown to exhibit uniform amplification characteristics with a gain excursion of ± 1 dB over a bond width of 34 mm with bg 980 mm pump wave length and a gain excursion of ± 1 dB over 30 mm in the c-bond operation regime with using 1480 mm pump wavelength.
In the subject invention the process doped fiber of optical amplifier consist of two highly asymmetric cores separated by a depressed cladding with reflective erbium-doping in the outer core including the cladding.
In the subject invention, the main advantage of new stair-care fiber structure over the coaxial fiber is the use of fiindamental mode at the pump wave-length as compared to the LP02 mode at 980 mm in case of the coaxial fiber, which requires an additional component (fiber brag grating) for LPOl to LP02 mode coupling.
Accordingly the present invention relates to an intrinsically gain flattened Erbium Doped fiber an amplifier comprising an inner cone having relatively large A, an outer
concentric depressed core having a smaller a cladding for covering the outer core wherein the bashed region e