Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-10-05
2001-07-24
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Polarization without modulation
C385S012000, C359S199200, C359S199200
Reexamination Certificate
active
06266457
ABSTRACT:
BACKGROUND
This invention relates in general to the field of telecommunications and fiber optic transmission systems and more particularly to a system and method for differential group delay compensation.
Fiber optics 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, which may be expressed in terms of a digital bits per second (“bps”) rate, has escalated. In some cases, for example, bandwidth has increased from 500 Mbps to 10 Gbps.
In a fiber optic transmission system, a digital signal is represented by an optical signal 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.
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, and dispersion are a few 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 optical link or channel can maintain a Bit Error Rate (“BER”) of 10
−13
or better. When an optical channel degrades to a BER of 10
−8
, a telecommunications system may automatically switch to an alternate optical channel in an attempt to improve the BER. Otherwise, the optical channel must operate at a reduced or lower bandwidth, which harms overall system performance. Dispersion is a major contributor to distortion of an optical signal, which increases the BER of the optical signal or channel. The distortion caused by dispersion generally increases as the bandwidth or data rate increases and as the optical fiber transmission distance increases.
Dispersion has generally been identified as being caused by (1) chromatic dispersion, or (2) Polarization Mode Dispersion (“PMD”). Until relatively recently, chromatic dispersion received the far greater attention because its adverse effects were initially more limiting at the then available bandwidth and data rate that was considered the leading edge in a fiber optic transmission system. Now, 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.
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.
Some of the major milestones, breakthroughs, and improvements that solved and/or reduced many of the problems caused by chromatic dispersion have included: (1) single-mode propagation, (2) Distributed Feedback (“DFB”) laser with narrow output spectra, and (3) the development of low attenuation/modified-dispersion optical fiber. All of these advances have contributed to increased bandwidth by allowing an optical signal to pass through an optical fiber with relatively low or reduced dispersion, and hence, relatively low or reduced optical signal distortion.
Single-mode propagation was achieved through the development of single-mode optical fiber. This optical fiber allows only a single mode of light to propagate through the fiber. The DFB laser provides a light source to use with single-mode optical fibers. The DFB laser produces a light with an extremely narrow distribution of output frequencies and wavelengths. This minimizes the chromatic dispersion problem caused by the fact that different wavelengths travel at slightly different speeds through a fiber. The low attenuation/modified-dispersion optical fiber provides a dispersion-shifted optical fiber that minimizes the speed-vs-wavelength dependency at a specific wavelength, such as 1550 nm.
Unfortunately, no corresponding major milestones, breakthroughs, and improvements have been achieved to solve and/or significantly reduce the substantial problems and limitations caused by PMD. PMD was previously insignificant relative to the other dispersion effects but now is a limiting factor. As a result, PMD now serves as a major limitation to the continued advancement and improvement in bandwidth and data rates for fiber optic transmission systems. PMD causes, among other problems, a first order effect that is referred to as Differential Group Delay (“DGD”). DGD refers to the fact that the two polarization states or modes of an optical signal, which are orthogonal to one another, are delayed relative to each other resulting in a leading polarization signal and a trailing polarization signal. This delay distorts the optical signal and limits the ability of a fiber optic transmission system to operate at a higher bandwidth and data rate.
It is well known that light can be polarized and that, for a given beam of light, this polarization may be expressed in terms of two orthogonal axes that are normal to the axis of propagation. Each of the two principal polarization modes may be expressed as polarization signals. As an optical signal or beam of light propagates through an optical fiber, birefringence causes the two polarization signals to travel or propagate at different speeds. This results in one of the two polarization signals leading the other polarization signal. Thus, there becomes a leading and a trailing polarization signal. As with chromatic dispersion, this speed difference in the two polarization signals causes pulse broadening and restricts the usable bandwidth of each optical carrier.
In many optical fibers, not only is birefringence present, but the birefringence is nonuniform and is randomly varying throughout the optical fiber. The PMD within a given optical fiber changes as a function of time, temperature, and various other factors. This results not only in the two polarization signals traveling at different speeds, but in the continual realigrnment or reorientation of the principal states of polarization. This is because the orientation of the refractive index in the optical fiber randomly or continually changes as the opti
Cherry Euncha
MCI Worldcom, Inc.
Spyrou Cassandra
LandOfFree
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