Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-06-30
2003-06-24
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06583906
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the fields of broadband cable television systems and is most closely related to laser optical communication links for such systems.
BACKGROUND OF THE INVENTION
In a cable television system, television programs are provided at a central head-end. The programs are distributed from the head-end through optical fiber tree networks to multiple local nodes in respective communities, and then further distributed from the local nods through coaxial cable tree networks to customer interface units (CIUs) also called cable terminations. Currently, many of these systems are beginning to provide other communication services such as telephone service and/or computer networking services (e.g. internet connection) through the cable television system. Telephone and computer networking services require bi-directional communication in the cable television system. Forward communication signals are transmitted, as described above for television program signals from the head-end to the customer interface units, and return communication signals travel the same path in the opposite direction. The return signals are collected from the CIUs through the coaxial cable networks to the local nodes, and further collected from the local nodes through the optical fiber network to the head-end.
At the head-end, a multitude of electronic signals for the programs and other communication services are used to modulate respective carrier signals with different respective frequencies. The modulated carrier signals are combined together into an electronic forward signal that is used to modulate a laser beam to produce an optical forward signal. The modulated laser beam, carrying the optical forward signal, is transmitted through an optical fiber tree network to a multitude of local nodes. At each local node, an optical detector coverts the optical forward signal back into an electronic forward signal. Then the reconverted electronic forward signal is transmitted through a coaxial conductor tree network to CIUs at homes and businesses of customers.
Telephone systems and computer systems connected to the CIUs by customers, produce return communication signals that are transmitted by the CIUs into the coaxial network. The return signals are multi-carrier modulated signals similar to the forward signals. The return signals travel back through the coaxial network to the local nodes. In the local nodes the return signals are separated from the forward signals by diplex filters. The separated return signals are used to modulate a return laser beam to produce an optical return signal carried by the return laser beam. The optical return signal is transmitted through an optical fiber network to the head-end where the optical return signals are converted back into electronic return signals by an optical detector. The electronic return signals are demodulated and used for telephone and computer communications.
Laser diodes are used to produce the laser beams that are modulated to convert the electronic signals into optical signals at the head-end and at the local nodes. In a directly modulated laser diode, the intensity of the laser beam depends on the current applied to the laser diode. The laser produces a signal as long as the current through the diode is positive and above a cutoff current level for the diode. Below the cutoff current level, the intensity of the laser is non-linear and falls quickly to zero. The current through the laser diode is modulated so that a modulation signal is carried by the laser beam. In order to produce a continuous signal, that is not cut off every time the signal becomes negative, the modulation signal is biased (e.g. a bias current is modulated by the modulation signal) so that the intensity of the laser beam produced by the laser is continuously modulated and negative portions of the signal are not lost. The electronic information signal includes positive and negative excursions of amplitude and the extent of some of the excursions are larger than other excursions. The bias is set so that the minimum amplitude of the biased electronic signal during the largest negative excursions of the signal is equal or higher than the cutoff bias of the laser diode.
The modulation index is the ratio between the power of the modulation of the laser beam and the total power of the laser beam. Thus, the modulation index is a measure of the energy efficiency of the communication so that increasing the modulation index reduces the energy required for the optical communications. In addition it has been found that the signal to noise ratio (SNR) is approximately proportional to the modulation index.
In order to transmit information without loss, it is critical to maximize the SNR. There are strict specifications for minimum SNR for all types of communications equipment, and the SNR requirements limit the distance through which signals may be transmitted through optical fiber links and coaxial cable links to customers. Generally, the noise in each stage of the communication system is additive to reduce SNR.
Those skilled in the art are directed to U.S. Pat. No. 4,941,208 to Olshansky in which a multitude of signals modulated by carriers of different frequencies are combined into a multi-carrier signal in which the sum of the modulation indexes of the signals is greater than one.
The above references are hereby incorporated herein in whole by reference.
SUMMARY OF THE INVENTION
In the invention herein, at a first node, an output electronic information signal is used to modulate a laser beam resulting in an optical information signal that is transmitted through an optical fiber to another node where, an optical detector converts the optical information signal into an input electronic information signal. The output electronic information signal includes high frequency positive and negative excursions of with respect to an average amplitude of the signal, and the extent of some of the excursions are larger than other excursions.
Prior to modulating the laser beam, the output signal is preprocessed to improve the resulting optical information signal. The preprocessor includes pre-shaper that transforms the electronic information output signal to reduce the extent of the larger negative excursions, so that, the modulation index can be increased for increasing the signal to noise ratio and increasing the energy efficiency of the communications. The transformed output signal is biased so that the current level in the biased signal is generally above a predetermined current level (e.g. the cut off current level of a laser), and the biased transformed signal is used to modulate the laser beam to provide an optical information signal.
Preferably, the output signal is a multi-carrier signal including a multitude of carrier signals each of a different frequency and each modulated by a respective baseband information signal.
The transformation may be a simple clipping of large negative excursions which provided continuous output of the laser. Clipping of large positive excursions may provide reduced noise if the signal with such positive excursions that are distorted are more noisy than the clipped and approximately restored signals. If post shaping is provided, cut off peaks can be estimated based, for example, typical shapes of cut off excursions based on the width of the clipped portion and/or on the derivative of the signals to the beginning and end of the cut off portion. More preferably, the transformation is a function selected to minimize third order distortions due to using the output signal to modulate a laser beam and due to transmitting the modulated laser beam through an optic fiber. Preferably, the transfer function is a parabolic transfer function. Also, the modulation index can be further increased by selecting a transfer function which reduces the extent of larger positive excursions with respect to the other excursions in the output signal so as to further increase the modulation index and further reduce distortion and noise resulting from modulating the las
Mutalik Venkatesh G.
Schemmann Marcel F.
Pascal Leslie
Tran Dzung
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