Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
1999-10-04
2004-06-15
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S173000, C398S176000, C398S177000, C398S178000, C398S147000, C359S341430, C359S333000, C359S337000
Reexamination Certificate
active
06751421
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical fiber communication systems and, in particular, to communication systems employing wavelength converters to enhance the bandwidth of transmission.
BACKGROUND OF THE INVENTION
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical transmission waveguide for carrying the optical signals, and a receiver for detecting the signals and for demodulating the information they carry. Typically the transmission waveguide comprises a plurality of segments of optical fiber interconnected by optical components such as rare-earth doped fiber amplifiers. Increasingly, the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
An important limitation on conventional fiber systems is the limited bandwidth of the optical components used in long distance transmission. While silica optical fibers have a wide bandwidth window of low loss transmission, some optical components intermediate fiber segments, such as erbium-doped fiber amplifiers, have more narrow bandwidths preventing full utilization of the transmission fiber window.
Silica optical fibers have an absorption coefficient less than 0.4 dB/km for wavelengths between 1250 and 1650 nm, making silica fibers suitable for long haul transmission over this entire spectrum. However current systems are typically limited to the wavelength range of 1530-1560 nm, where conventional erbium-doped silica fiber amplifiers (EDFAs) perform well. While prototypical EDFAs have been demonstrated over the wavelength range from 1530-1610 nm (see for example A. K. Srivastava et al., “1 Tb/s Transmission of 100 WDM 10 Gb/s Channels Over 400 KM of TrueWave™ Fiber”, OFC '98 Post Deadline Paper PD10 (1998)), it is doubtful that the operating range of EDFAs will be expanded over a much wider wavelength range.
It is common for lossy elements, such as dispersion compensating fiber, gain flattening filters and variable attenuators, to be included within discrete amplifiers. The loss in these elements can exceed 20 dB such that an EDFA with a 25 dB external gain will have an internal gain of 45 dB. The noise figure of these amplifiers is typically less than 6 dB. Amplifiers will need to meet such requirements in order to be practical for many applications.
One alternative amplifier is the Raman amplifier. This amplifier can provide gain at any wavelength and has been demonstrated at 1300 nm and in the 1500 nm range (see for example P. B. Hansen et al., “High Sensitivity 1.3 &mgr;m Optically Preamplified Receiver Using Raman Amplification”,
Electron. Lett
., Vol. 32, p.2164 (1996) and K. Rottwitt et al., “A 92 nm Bandwidth Raman Amplifier”, OFC '98 Post Deadline Paper PD6 (1998)). Disadvantageously, Raman amplifiers require high pump powers. This is particularly true for high gain amplifiers.
Another alternative amplifier is a parametric amplifier (see for example E. Desurvire, Erbium Doped Fiber Amplifiers, p.451) (Wiley, 1994). These amplifiers are typically based on four wave mixing (FWM). They have the disadvantages of requiring very high pump powers and of requiring precise control of the fiber dispersion in order to achieve phase matching over long lengths of fiber.
Four wave mixing (FWM) can also be used for wavelength conversion and spectral inversion. Proposed applications of this technology include wavelength routers (S. J. B. Yoo, “Wavelength Conversion Technologies for WDM Network Applications”,
J. Lightwave Technology
, Vol. 14, p. 955 (1996)), optical switching, and mid-span spectral inversion (S. Watanabe et al., “Exact Compensation for Both Chromatic Dispersion and Kerr Effect in a Transmission Fiber Using Optical Phase Conjugation”,
J. Lightwave Technology
, Vol. 14, p. 243 (1996)). Communication systems have been demonstrated that employ FWM for spectral inversion over broad bandwidths (e.g. >70 nm) but without signal amplification (conversion efficiency of ~−16 dB) (S. Watanabe et al., “Interband Wavelength Conversion of 320 Gb/s WDM Signal Using a Polarization-Insensitive Fiber Four-Wave Mixer”, ECOC '98 (1998)). Using small effective area fibers, conversion efficiencies of up to 28 dB over 40 nm (G. A. Nowak, et. al., “Low-Power High-Efficiency Wavelength Conversion Based on Modulational Instability in High-Nonlinearity Fiber”,
Opt. Lett
., Vol. 23, p.936 (1998)) are possible, however pump powers of 28 dBm are required. Single channel gain of ~0 dB has been reported at pump power of 17 dBm in standard dispersion shifted fiber, however the fiber loss resulted in net loss of the converted signal (S. Watanabe et al., “Highly Efficient Conversion and Parametric Gain of Nondegenerate Forward Four-Wave Mixing in a Singlemode Fibre”,
Electron. Lett
., Vol. 30, p. 163 (1994)). The results to date indicate that parametric amplification alone in silica fiber using pump powers less than 30 dBm will not be able to provide the gain needed for a discrete amplifier in a conventional terrestrial communications systems. Accordingly there is a need for a new kind of optical communication system for broadband transmission.
SUMMARY OF THE INVENTION
The present invention uses wavelength conversion to increase the bandwidth of optical communication systems. In an exemplary embodiment, a combination of wavelength conversion and amplification with a discrete optical amplifier (OA) to allow communications systems to operate in wavelength bands &lgr;′ outside the gain bandwidth of the OA. A transmitter launches signal channels (&lgr;
1
′, &lgr;
2
′, . . . &lgr;′
N
) that are outside the gain bandwidth &lgr;. A wavelength conversion device upstream of the amplifier maps channels &lgr;′
1
, &lgr;′
2
, . . . &lgr;′
N
to corresponding wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
within &lgr;. The OA directly amplifies the converted signals and a second wavelength conversion device downstream of the amplifier maps the amplified signals back to the original channels &lgr;′
1
, &lgr;′
2
, . . . &lgr;′
N
. This increases the capacity of the optical communication systems by facilitating the use of both signals that lie within the OA gain bandwidth &lgr; and signals that can be converted to wavelengths within &lgr;. Associated wavelength converters, transmitters and receivers are also described.
This approach applies not only to the use of EDFAs, but also to gain-flattening elements, dispersion-compensating fibers, variable attenuators, and any intermediate components having bandwidths smaller than the transmission fiber.
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Ranka Jinendra Kumar
Stentz Andrew John
Lucent Technologies - Inc.
Pascal Leslie
Phan Hanh
LandOfFree
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