Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-01-28
2002-04-02
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S024000, C359S199200
Reexamination Certificate
active
06366728
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of telecommunications and fiber optics and more particularly to a composite optical fiber transmission line and method.
BACKGROUND OF THE INVENTION
Advances in fiber optic technology and fiber optic transmission systems are revolutionizing telecommunications. The main driving force behind this revolution is the promise of extremely high communications bandwidth. A single beam of modulated laser light can carry vast amounts of information that is equal to literally hundreds of thousands of phone calls or hundreds of video channels. Over the past few years, this technology has advanced at such a pace that the bandwidth capabilities have more than doubled every two years. The bandwidth strides have come about through major milestones, breakthroughs, and improvements in various areas such as fiber optic materials and transmitter devices. As a result, bandwidth capability or data rates, which may be expressed in terms of digital bits per second (“bps”), have escalated. In some cases, for example, capacity has increased from 500 Mbps to 10 Gbps and higher.
In a fiber optic transmission system, a digital signal is represented by an optical signal. The optical signal is generated by modulating a laser light or rapidly turning a laser light on and off to represent the various “1” and “0” or “on” and “off” values or states of a digital signal. This may be referred to as amplitude modulation. The laser light, or optical signal, is generally emitted from a laser of an optical transmitter. In the frequency domain, this signal includes numerous frequency components spaced very closely about the nominal center frequency of the optical carrier, such as, for example, 193,000 Ghz.
To increase the overall data rate or bandwidth of a fiber optic transmission system, multiple optical signals may be multiplexed using Wavelength Division Multiplexing (“WDM”) or Dense Wideband Division Multiplexing (“DWDM”). WDM and DWDM both involve simultaneously transmitting two or more optical signals, each at a different wavelength or frequency, through an optical fiber in the same direction.
WDM has been used to refer to multiplexing or combining two or four optical signals, while DWDM has been used to refer to multiplexing or combining eight, sixteen, and even forty or more optical signals. Each wavelength of a WDM or DWDM optical signal is a virtual optical channel that may support, for example, data rates of OC-48 at 2.5 Gbps or OC-192 at 9.953 Gbps to provide a significant overall data rate. Optical Erbium-Doped Fiber Amplifiers (“EDFA”) are typically used at repeaters to simultaneously and directly boost all wavelengths or virtual optical channels of such WDM or DWDM optical signals. This provides the tremendous advantage of eliminating the requirement of separating each WDM or DWDM optical signal into its various optical signals of different wavelengths, converting each such optical signal to its electrical signal equivalent, amplifying each such electrical signal, and then combining or multiplexing the various signals to regenerate the WDM or DWDM optical signal.
Unfortunately, WDM and DWDM may create wave mixing, such as four-wave mixing, between the various optical signal wavelengths of the WDM or DWDM optical signal. This can increase the Bit Error Rate (“BER”) of the optical fiber transmission system. Further, WDM and DWDM optical signals are provided at higher power levels and require optical fibers capable of handling the higher power levels. While it is difficult to design an optical fiber transmission system to accommodate an optical signal at a single wavelength, it is exceedingly more challenging and difficult to design an optical fiber transmission line that can simultaneously accommodate multiple wavelength optical signals, such as WDM and DWDM optical signals.
An optical signal is transmitted in a fiber optic transmission system using, generally, an optical transmitter, which includes a light source or laser, an optical fiber, an optical amplifier, and an optical receiver. A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Noise, attenuation, modal dispersion, chromatic dispersion, chromatic dispersion slope, polarization mode dispersion, and wave mixing are some of the impairments that can distort an optical signal and render the optical signal marginal or unusable at the receiver. The distortion of an optical signal makes it extremely difficult or impossible for an optical receiver to accurately detect or reconstitute the digital signal. This is because distortion nonuniformly broadens, spreads, or widens the various light pulses resulting in such closely spaced pulses or overlapping pulses that the pulses are virtually indistinguishable from one another.
Conventionally, a properly designed fiber optic transmission system or channel can maintain a BER of 10
−13
or better. When an optical channel degrades to a BER of 10
−8
, a telecommunications system automatically switches to an alternate optical channel in an attempt to improve the BER. Otherwise, the optical channel operates at a reduced or lower data rate or bandwidth, which harms overall system performance. All types of dispersion, modal, chromatic, and polarization mode, make the BER worse.
The negative effects of modal dispersion have been dramatically reduced and, in some cases, effectively eliminated through the use of single-mode fiber. Single-mode fiber prevents or reduces the ability of an optical signal to take multiple or different paths through an optical fiber. This prevents or reduces multimode distortion. Single-mode fiber allows only a single mode of light to propagate through the fiber. Single-mode fibers generally achieve this through the use of a smaller core, as compared to multimode fibers.
Chromatic dispersion and Polarization Mode Dispersion (“PMD”) remain major contributors to distortion of an optical signal, which increases the BER of the optical channel. The distortion caused by chromatic dispersion and polarization mode dispersion generally increases as the bandwidth or data rate increases and as the optical fiber transmission distance increases.
Chromatic dispersion and PMD have been identified as the major contributors to distortion. Chromatic dispersion has received the far greater attention because its adverse effects were initially more limiting at the then available bandwidth and data rate of the leading edge in optical fiber transmission systems. More recently, it has been recognized that PMD is one of the limiting factors that must be overcome to take telecommunications and fiber optic transmission systems to the next level and to continue with the heretofore rapid increase and expansion of bandwidth and data rates. Developments have been made and continue to be made to address problems and limitations caused by PMD.
Chromatic dispersion occurs when the various frequency components or colors that make up a pulse of laser light travel at different speeds through an optical fiber and arrive at the optical receiver at different times. This occurs because the index of refraction of a material, such as an optical fiber, varies with frequency or wavelength. As a result, the various pulses of light that make up an optical signal are distorted through pulse spreading, making it difficult or impossible to accurately receive and recover the digital data contained in the optical signal. Chromatic dispersion presents problems when it is too high and when it is too low. In addition to a distorted optical signal and a reduced data rate, high chromatic dispersion may also result in self phase modulation and generally requires the use of a long dispersion compensating fiber. When chromatic dispersion is too low, the problem of cross-phase modulation may present significant limitations. Thus, it may not be desirable to completely eliminate chromatic dispersion in all cases. Chromati
Li Yisong
Way David G.
Xia Tiejun
MCI WorldCom Inc.
Palmer Phan T. H.
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