Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2002-01-15
2004-08-24
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S197000
Reexamination Certificate
active
06782205
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of signal equalization within wavelength division multiplexed optical transmission networks. More particularly, the present invention relates to the use of diffractive light modulators for signal equalization of component signals within wavelength division multiplexed optical fiber networks.
2. Background of the Invention
Increasingly, data transmitted through the telecommunications network is shifting from an electrical data transmission to optical data transmission. Design parameters for fiber optical networks seek to create an architecture which allows the highest number of bits per second in transmission, while simultaneously reducing costs by developing a system that affords the greatest distance between repeaters, and maintaining system reliability where transmission errors are held to an acceptably low level. These three operational characteristics are oftentimes adverse to each other. Signal strength, that is, the power of the signal, attenuates during transmission, and the greater the transmission distance, the greater the attenuation. As signal strength decreases, the signal to noise ratio decreases. The bit error rate increases exponentially as the signal to noise ratio decreases. The need for repeaters is therefore governed largely because of line loss or attenuation of an optical signal through an optical medium. At some critical point in attenuation, a signal under transmission will become be too weak to reproduce reliably, and the bit error rate will climb to unacceptable levels. Accordingly, a lower threshold for signal strength is established as a system parameter of an optical network to ensure system reliability. To operate within such a parameter, the system must be designed to prevent a signal from falling below the lower threshold. Typically, signal attenuation is measured in dB per km. Accordingly, repeaters must be spaced close enough together that a signal has not attenuated below the lower threshold by the time it reaches the next repeater, at which point it is either amplified, or processed in some other manner for re-transmission. Because it is economically dis-advantageous to have repeaters spaced more closely together than necessary, repeaters are typically placed near the maximum distance at which a signal can be reliably received and processed for accurate reconstruction.
In endeavoring to maximize the distance between repeaters, an increase in the power of a signal will therefore increase the distance that a signal may reliably travel before falling below the lower threshold. It is easily understood, however, that there is an upper limit of allowable power when transmitting a signal across an optical network. Above that upper limit, an increase in signal strength is at best superfluous, and at worst, maintenance intensive, economically prohibitive or even deleterious to the integrity of the optical network. The upper threshold of signal transmission power is therefore another operational parameter for a fiber optical network.
Analog and digital communication have long used frequency/wavelength multiplexing as one means of achieving greater bandwidth. Through multiplexing, discrete signals defined by distinct wavelengths are transmitted across the same medium. Each discrete signal is typically assigned to carry specific information. Signal attenuation within a fiber network, however, is oftentimes frequency or wavelength dependent. Accordingly, the rate of attenuation, commonly measured in dB/km can vary among different wavelengths within a fixed optical spectrum. Consequently, the wavelength distinguished by the highest rate of attenuation will typically govern fundamental network parameters such as the maximum distance between repeaters. One result of differing rates of attenuation, therefore, is that different wavelengths transmitted at a same power will be at different power levels upon reaching a repeater or other processing station. In addition, the routing and switching of signals within a metropolitan network has the capacity to combine signals of disparate power levels. Moreover, there is unevenness in the multiplexing and demultiplexing components, unequal gain over different wavelengths in erbium doped fiber amplifiers (EDFAs), unequal laser launch power for the different channels, etc. All of these features exacerbate the uneven power levels of different wavelengths during the transmission, re-transmission, routing and processing of an optical signal.
FIG. 1
illustrates a spectrum made up of many discrete wavelengths, from a first wavelength &lgr;
1
up to an n
th
wavelength &lgr;
n
, which form component signals within a collective wavelength multiplexed signal within an optical medium. The Y-axis represents signal power, and the X-axis represents a spectrum of wavelengths. It is commonly understood by those skilled in the art that wavelength and frequency are inversely proportional. These terms may therefore be used interchangeably throughout to distinguish component signals. Moving along the X-axis is therefore equally understood to represent a spectrum of frequencies. The lower signal threshold
124
is the lowest signal power level to which a signal may attenuate and remain reliably processable according to system requirements. The “saturation threshold”
120
is the maximum allowable signal power of the network for any one wavelength. Between these two levels, a reference power level
122
is illustrated throughout
FIGS. 1
,
2
and
4
for comparative purposes only. For illustrative purposes, it is assumed that all of the component wavelengths or frequencies depicted in
FIG. 1
began at equal signal strength, and have attenuated to the levels seen in
FIG. 1
during launch, transmission, routing or other processing within a fiber optical network. As seen in
FIG. 1
, the signals can be at different strengths. The third wavelength &lgr;
3
is seen to be quite robust, remaining above the reference level
122
. Contrariwise, the fourth wavelength &lgr;
4
is seen to have attenuated to a signal strength substantially below the reference level
122
.
FIGS. 2 and 3
show the signals of
FIG. 1
after each component wavelength has been uniformly amplified. Because the third wavelength &lgr;
3
was the strongest signal prior to amplification, it remains the strongest signal after amplification. Plotting uniformly amplified signals, the relationship in signal strength is therefore unchanged from the pre-amplification relationship of
FIG. 1
, provided all of the component signals remain below the saturation threshold.
FIG. 2
shows all component signals within the upper limit of the network parameters, with the strongest signal, the third wavelength, &lgr;
3
, at the upper limit. As noted however, the other discrete wavelengths fall far below the upper threshold. Because it was earlier determined that the fourth wavelength &lgr;
4
was subject to the greatest attenuation during transmission, future transmission subsequent
FIG. 2
is limited by the fourth wavelength &lgr;
4
, which is both the weakest signal, and subject to the greatest attenuation. Failure to amplify the fourth wavelength &lgr;
4
to the maximum allowable signal strength
120
will result in attenuation of &lgr;
4
to the lower threshold
124
in a substantially shorter transmission distance than if it had begun at the upper threshold
120
. Alternatively,
FIG. 3
shows the fourth &lgr;
4
, which is the weakest component wavelength in the figure, amplified to the upper threshold
120
. The problem with this approach, however, becomes clear when an examination is made of the other component signals in FIG.
3
. By amplifying the weakest signal up to the upper threshold
120
of the network, in a uniform amplification process, all other signals, &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
n
are amplified above the upper threshold
120
of the optical network.
To optimize network performance therefore, a first step in the processing of a wavelength multiplexed signal is chann
Carlisle Clinton B.
Trisnadi Jahja I.
Haverstock & Owens LLP
Leung Christina Y
Pascal Leslie
Silicon Light Machines
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