Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-04-27
2003-03-04
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S337500
Reexamination Certificate
active
06529315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to optical amplifiers, and more particularly, to Raman optical amplifiers providing dispersion compensation.
2. Description of Related Art
In long haul optical communications systems it is desirable to maximize an optical fiber's capacity to carry information (i.e. maximize the aggregate number of bits per second that can be transmitted on the fiber) and to maximize the distance that the signals can propagate in the optical domain before it is necessary to resort to expensive optoelectronic regenerators.
Dense Wavelength Division Multiplexing (DWDM) is a technology for optical communications which uses densely packed wavelengths of light to effectively multiply the capacity of the optical fiber. Each wavelength carries a distinct signal. The performance of such systems is limited by optical attenuation which progressively weakens the optical strength of the signals as they propagate along the fiber. DWDM optical communications systems are practical because of the use of optical amplifiers which restore the strength of signals of all wavelengths simultaneously, to counteract the effects of optical attenuation. Amplifiers are typically selected to provide enough amplification to restore the signal but not more than necessary to restore the signal. Too much amplification would upset the gain balance, and can lead to impairment to signal transmission.
The most commonly deployed optical amplifier is an Erbium-Doped Fiber Amplifier (EDFA). An EDFA amplifies wavelengths of light within a large frequency band (~4 THz for a conventional EDFA at the time of this writing). However, although this frequency space is large, it is relatively small compared with the total bandwidth of the low loss window of the optical fiber. Thus, the EDFA bandwidth generally restricts the usable bandwidth (BW). A typical conventional band (C-band) EDFA operates from approximately 1528 to approximately the 1563 nm range. L-band EDFAs operate approximately from 1567 to the 1605 nm range. It is a fundamental property of optical amplifiers that in addition to delivering signal gain which strengthens the signals, they also produce noise (in the form of amplified spontaneous emission, ASE) which degrades the signal.
For economic reasons, it is desirable that the lengths of transmission fiber between these optical amplifiers be as large as possible. However, the further the signals must travel from one optical amplifier to the next, the more the signals weaken due to optical attenuation, and the more severely the noise added at each amplifier degrades the signal. The distances over which such signals can be transmitted are generally limited by the accumulation of such noise.
When such optical noise is the most important impairment, the quality of the signal at the end of the system can be improved by increasing the optical power produced by each optical amplifier. In practical systems, the ability to increase optical power is constrained; specifically because when the optical powers of signals in the channels in the fiber exceed a certain level, they create optical nonlinear effects (such as four wave mixing, self phase modulation and/or cross phase modulation) which distort the signals and impair their quality. Thus, it is very important to minimize the impairments arising from optical noise without increasing the optical power beyond the nonlinear limit.
In addition to increasing the capacity of optical fibers to carry signals by using DWDM technology, their capacity is also increased by Time Division Multiplexing (TDM) (i.e. multiplexing time-tributary signals at lower bit rates into multiplexed aggregated signal streams at higher bit rates which are transmitted over the fiber as a single serial stream of bits at the aggregated rate). The extent of such multiplexing is limited in part by the ability to process, produce and detect such high speed Time Division Multiplexed signal streams but even more so by the ability of such very short, very frequent pulses to maintain their integrity while propagating along long lengths of transmission optical fiber.
The most severe impairment limiting the data rate of TDM signal channels is chromatic dispersion. Chromatic dispersion is a property of the optical fiber which causes light of different wavelengths to propagate at different speeds. Any optical pulse is made up of light of a range of wavelengths and the shorter the pulse, the wider the range of wavelengths which make up the pulse. In the presence of chromatic dispersion, these wavelengths propagate at different speeds and this causes the pulse to spread out in time, with the longer wavelength components of the pulse trailing further and further behind its faster, shorter wavelength components as the pulse propagates down the fiber. The signal is impaired when the pulses spread sufficiently that they overlap with the neighboring preceding or subsequent pulses and can no longer be distinguished by the receiver.
The chromatic dispersion of silica, the material from which most optical fibers are made, is zero at a wavelength of about 1300 nm. At this wavelength, conventional single mode fibers, which are widely deployed, also have zero chromatic dispersion. Near 1550 nm, where transmission fibers have their lowest optical loss and where optically amplified systems using EDFAs operate, such fibers have high dispersion, typically 17 ps
m/km. For such fibers, the typically 100 ps wide pulses used in 10 Gb/s systems spread in time so quickly as they propagate that data cannot be transmitted further than about 50 km before electrical regeneration of the pulses becomes necessary.
During the 1980's, Dispersion Shifted Fibers (DSF) which have zero dispersion near 1550 nm were developed and widely deployed in some networks because of the potential to support higher data rates with very low dispersion. However, such fibers cannot support DWDM transmission because the impairments that result from nonlinear interactions between the different wavelength channels (primarily four wave mixing but also cross phase modulation) are more severe when the dispersion is small and the different channels travel at similar speeds. For this reason, for high capacity systems which combine high channel multiplicity dense wavelength division multiplexing with high data rate TDM on each of the WDM channels, the preferred transmission fiber is so-called Non-Zero Dispersion Shifted Fiber (NZDSF). In NZDSF the dispersion in the wavelength region of interest near 1550 nm ranges typically from about 2 to 4 ps
m/km, large enough so that the nonlinear interactions among channels will not unduly impair the signal quality. However, the dispersion of NZDSF, while less than that of standard single mode fiber, is still large enough that for long-haul transmission (i.e., several hundred km or more) the dispersion-induced pulse broadening will limit the transmission distance for 10 Gb/s systems; for the higher speed 40 Gb/s systems, the limitations will be sixteen times more stringent.
Dispersion compensation, which reverses the impairment caused by dispersion, is a key technology for the transmission of high-speed TDM signals (i.e., 10 Gb/s, 40 Gb/s and more) over standard single mode fiber and over NZDSF. Dispersion-compensating fiber (DCF) consists of a fiber specially designed to have chromatic dispersion with a sign opposite to that of conventional single mode fibers (i.e., light with longer wavelengths travels faster than light with shorter wavelengths). Pulses which have been dispersed (i.e., broadened in time) by propagating over dispersive optical fiber can be narrowed to their original width, restoring the integrity of the signal, by traversing a DCF the length of which is chosen so that the faster traveling light of longer wavelengths—which are the slower wavelengths in the transmission fiber - exactly catch up to the light with shorter wavelengths which had left them behind.
In the absence of distortion arising from optical nonlinearities, compensation for dis
Bartolini Glenn
Pendock Graeme
Zyskind John
Hellner Mark
Sycamore Networks, Inc
Wolf Greenfield & Sacks P.C.
LandOfFree
Optical amplifier providing dispersion compensation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical amplifier providing dispersion compensation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical amplifier providing dispersion compensation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3026813