Optical: systems and elements – Optical amplifier – Particular pumping type
Reexamination Certificate
1999-08-12
2001-05-15
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular pumping type
C359S199200, C359S199200, C359S341430, C372S006000, C372S071000
Reexamination Certificate
active
06233092
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to optical amplifiers for high speed telecommunications systems, and more particularly, to an amplifier structure and associated method for efficient use of amplified spontaneous emission (hereinafter ASE) in a multistage, rare earth doped optical amplifier such as an EDFA, in which ASE is utilized for providing L-band optical signal amplification (hereinafter referred to as “ASE pumping”).
DESCRIPTION OF RELATED ART
Optical telecommunications service providers continue to demand more data capacity and higher data transmission speeds to service their customers' current and future requirements. In systems employing EDFAs, however, channel density has been limited by the usable gain bandwidth of the EDFA. This bandwidth is on the order of 35 nm even when gain flattening filters are used to flattened the erbium gain spectrum for multichannel amplification. Three techniques for increasing system capacity in multichannel lightwave systems include (1) increasing bit rate per channel, (2) increasing the number of channels by decreasing channel spacing, and (3) increasing number of channels by increasing the total gain/transmission bandwidth of the gain media. Increasing the bit rate per channel is not always a viable solution as many installed systems cannot operate beyond the current OC-48 bit rate (2.5 Gb/s). Likewise, fiber nonlinearities limit reducing channel spacing below the current values of 50 GHz-100 GHz. Thus increasing the EDFA gain bandwidth allows a direct path for increasing system capacity while maintaining channel spacing and bit rate per channel. As far back as 1990, Ainslie et al., High gain, broadband 1.6 micron ER
3+
doped silica fiber amplifier,
Electronics Letters,
volume 26, pp. 1645-1646 (1990) investigated the long band (1565-1610 nm) utility of the erbium gain spectrum. Recently, Srivastava et al., 1 Tb/s transmission of 100 WDM 10 Gb/s channels over 400 km of Truewave fiber, Tech. Dig. OFC'98, Post deadline paper PD10-1, San Jose, Calif., 1998, demonstrated application of silica EDFAs in the 1.6 micron band (L-band). Sun et al., Ultrawide band erbium-doped silica fiber amplifier with 80 nm of bandwidth, PROC. OAA, Post deadline paper PD 2-2, Victoria, BC Canada, 1997, discussed a split band architecture that amplifies both the conventional C-band (1530 nm-1560 nm) and the L-band, providing a total gain bandwidth of 80 nm. Thus L-band amplification offers a demonstrable, but undeveloped, solution to the bandwidth constraints in WDM lightwave systems.
It is appreciated by those skilled in the art that EDFAs operating in the L-band typically have features that distinguish them from amplifiers designed to work in the heavily used C-band from approximately 1530 nm to 1560 nm. Notable differences that have been observed include relatively flatter gain spectra at low inversions (i.e., 0.4 versus 0.6-0.7); and required longer lengths of erbium doped fiber. These are due at least in part to the relatively low emission cross section of erbium for wavelengths greater than approximately 1560 nm.
The power conversion efficiency for L-band EDFAs has typically been lower than that observed in conventional C-band EDFAs. We believe this is related to the approximately 10 times greater emission cross section of erbium in aluminum co-doped silica at the emission peak near 1530 nm than at 1600 nm; thus there can be considerable emission at the shorter wavelength before the signals in the longer band have been substantially amplified. This is compounded by the fact that the spontaneous emission near the emission peak will have a higher gain coefficient than the signals themselves (assuming that the inversion of the input stage is greater than approximately 60%, as required to get the noise performance typically desired of EDFAs for communications systems) and, as a result, is amplified at a higher rate. Moreover, the unusually long erbium doped fiber lengths required for low inversion amplification (i.e., L≧100 meters for current typical Er concentrations) provides large amounts of reverse traveling ASE.
Armed with an appreciation of the fact that signals contained within the erbium L-band can be efficiently pumped with pump wavelengths extending into the C-band, which is due to the properties of low inversion amplification, the inventors have recognized the utility of using properly managed ASE as an effective pump for L-band gain.
It is therefore an object of the invention to provide an optical amplifier, e.g., a multi-band amplifier, for an optical telecommunications system having increased bandwidth capabilities.
It is another object of the invention to provide an optical amplifier having a significantly expanded bandwidth in comparison to conventional optical amplifiers used in WDM transmission systems.
It is a further object of the invention to provide an optical amplifier having an expanded amplification bandwidth that is pumped by ASE generated by the long wavelength band structure of the amplifier.
It is still another object of the invention to provide an optical amplifier as described above that has desirable noise performance and which exhibits a design that is pump power efficient.
SUMMARY OF THE INVENTION
In accordance with these objectives and others, the invention is broadly directed to an optical amplifier having a structure that efficiently utilizes ASE (typically the wavelength region 1525 nm-1565 nm) as a pump source, and as a consequence provides an increased signal bandwidth over conventional C-band amplifiers. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objective and other advantages of the invention will be realized and obtained by the apparatus and method particularly pointed out in the written description and claims hereof, as well as the appended drawings.
An embodiment of the invention is directed to an optical amplifier including first and second amplifier stages that are serially interconnected; and means for pumping either the first or the second amplifying stage in either a forward or a reverse direction for generating an amount of ASE from the pumped amplifier stage which is used for pumping the other, unpumped amplifier stage.
In another embodiment, an optical amplifier includes first and second amplifier stages that are serially interconnected by a coupler for propagating a signal light from the first amplifier stage to the second amplifier stage; and a pump light source coupled to the first amplifier stage via the coupler for introducing pump light in a reverse traveling direction into the first amplifier stage, wherein the reverse pumping of the first amplifier stage provides an amount of ASE traveling in a forward direction towards and into the second amplifier stage for providing L-band signal amplification in the second amplifier stage.
In another embodiment, an optical amplifier includes first and second amplification stages of erbium doped fiber which are serially interconnected by a coupler for propagating signal light from the first stage to the second stage and for coupling forward traveling pump light into the second amplification stage wherein an amount of ASE is generated in a reverse traveling direction from the second amplification stage to the fist amplification stage for causing L-band signal amplification in the first amplifier stage. In an aspect of this embodiment, the amplifier further includes means for receiving and redirecting an amount of unabsorbed ASE from the first amplifier stage back into the first amplifier stage.
In another embodiment, first and second serially connected rare earth doped amplification stages of lengths L
1
and L
2
, respectively; a source of pumping light coupled to an input of the first stage for providing a relatively high inversion and substantially complete absorption of the pump light in that stage and which provides an amount of ASE traveling back towar
Flood Felton Aaron
Yadlowsky Michael John
Corning Incorporated
Greener William
Moskowitz Nelson
Short Svetlana Z.
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