Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Utility Patent
1998-04-02
2001-01-02
Vu, Huy D. (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C375S345000, C455S232100, C455S234200, C455S235100, C455S239100, C330S136000, C250S2140AG
Utility Patent
active
06169619
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical signal receiving apparatus and method.
Along with the development of multimedia technology, optical communication using optical fibers has been rapidly spreading. Particularly, while optical communication hithertofore had mainly spread at the trunk line side, in the future optical communication will mainly spread at the subscriber side for realizing “fiber to the home” (FTTH) systems. The “subscriber side” referred to here includes not only individual subscribers (users), but also local area networks (LANs) etc.
In for example a passive optical network (PON) transmission system, the subscriber side is constituted by an optical splitter (star coupler) connected to the office side equipment via a common optical fiber transmission line and a plurality of units of subscriber side equipment individually connected to this optical splitter via individual optical fiber transmission lines. Here, downstream optical transmission from the office side equipment to each subscriber side equipment is carried out by continuously transmitting cell signals, while upstream optical transmission from the plurality of units of subscriber side equipment to the office side equipment is carried out by using time slots allotted to the units of subscriber side equipment in advance and transmitting the cell signals of each subscriber in a burst-like manner in a time division mode in units of cells each having a fixed bit length.
Looking at the latter upstream optical transmission, the office side equipment receives the bursts of cell signals from the optical transmission apparatuses of the units of subscriber side equipment. The present invention relates to an optical signal receiving apparatus which receives the burst-like cell signals in the office side equipment. This optical signal receiving apparatus receives cell signals having different levels of received light for every time slot due to the difference of transmission loss of the individual optical fiber transmission lines. Therefore, the automatic gain control (AGC) for this becomes important subjects.
2. Description of the Related Art
As will be explained in detail below referring to the drawings, there are the following problems in the related art.
Since the optical signal receiving apparatus receives the cell signals having different amplitude levels in bursts for every subscriber, the optical signal receiving apparatus has to instantaneously determine the optimum detection state for each amplitude of the incoming cell signals, that is, the optimum gain.
However, the optical signal receiving apparatus must be given a considerably large range of variation of gain control in order to cope with burst-like cell signals having a wide dynamic range. As a result, the instantaneous response of the optical signal receiving apparatus to the burst-like cell signals is deteriorated. In order to deal with this deterioration of the instantaneous response, it is necessary to secure a transient time region (period) until the gain stabilizes at the optimum value.
In the state where no cell signal has yet arrived, for example, the optical signal receiving apparatus waits on standby at the maximum gain. When a cell signal arrives during this standby state, the optical signal receiving apparatus first amplifies the input signal with its maximum gain and then lowers the gain if the level of the amplified signal is larger than the optimum value. It then waits for stabilization of the gain. Usually, at least 10 bits worth of time is necessary until the gain is stabilized. When the dynamic range of the cell signals becomes larger, the range of variation of the gain is enlarged and therefore the number of bits must be further increased. That is, the transient response region is increased.
Due to the above explained control of the gain at every time of reception of signal, there is sometimes an overshoot at each rising edge of the amplified signal from the later explained variable gain amplifier circuit. During the occurrence of this overshoot, the signal is not the inherent amplified signal, so it is difficult to correctly discriminate between the logics “1” and “0” at the following discrimination circuit.
The region of above difficulty of discrimination is the follow-up period up to the stabilization to the optimum gain, that is, a training period. In the related art, each cell signal has a data region for the training added to its front position as a header. A data region added as a header in this way does not contribute at all to the region of the cell signals for transfer of data as inherent information. In the end, therefore, there is a problem of a reduction of the transmission efficiency of the transmission system as a whole.
SUMMARY OF THE INVENTION
Accordingly, in consideration with the above problems, an object of the present invention is to provide an apparatus and method for detection of an optical signal capable of causing the gain to quickly stabilize at the optimum gain without accompanying transient response (overshoot) at the time of reception of each cell signal.
To attain the above object, the present invention provides an optical signal receiving apparatus capable of substantially instantaneously and stably discriminating the logic from the header of a burst-like optical signal. This apparatus comprises a pre-amplifier circuit for amplifying an output from a photodiode receiving cell signals, a dummy side variable gain amplifying unit for performing the gain control by feedback control with respect to the output from the pre-amplifier circuit and outputting the gain at that time as gain information, a memory unit for holding the gain information, and a signal side variable gain amplifying unit for performing the gain control by feedfoward control by using the gain set by the gain information and outputting the amplified signal.
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Fujitsu Limited
Lieu Vu
Staas & Halsey
Vu Huy D.
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