Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-23
2001-02-20
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C250S2140AG, C330S059000, C375S317000
Reexamination Certificate
active
06191879
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical receivers, and in particular to automatic offset control for an optical receiver for receiving a burst mode optical signal.
2. Description of the Related Art
There has been an optical receiver without the use of coupling capacitor, which is generally called a dc-coupled receiver. Although such a dc-coupled receiver is ideally suitable for burst mode operation, it is very important to solve the offset problem stemming from faulty or deteriorated extinction ratio and reflected return light at sending side and a dark current flowing through a photodiode at receiving side. If such an offset current is not compensated for, the receiving sensitivity of the receiver would be impaired.
To compensate for the offset, an automatic offset canceler (AOC) has been proposed (see Japanese Patent Unexamined Publication NO. 5-218772). In the AOC, a differential amplifier produces a positive output signal and a negative output signal from an input signal. Positive and negative peak detectors receive the respective positive and negative output signals to produce positive and negative peak voltages which are used to produce an offset adjustment signal based on a difference voltage thereof.
Another conventional AOC has been disclosed in U.S. Pat. No. 5,539,779 (issued on Jul. 23, 1996). This conventional AOC is comprised of a differential preamplifier, an average detector, a peak detector, and a differential amplifier. The average detector detects an average voltage of a positive output signal and a negative output signal of the differential preamplifier. The peak detector detects a peak voltage of the negative output signal of the preamplifier. The differential input amplifier compares the peak voltage with the average voltage as a reference value to produce an offset adjustment signal. In other words, the offset adjustment signal is obtained based on a difference between the average voltage and the peak voltage.
SUMMARY OF THE INVENTION
The inventor found that the above offset canceler can sufficiently compensate for a dc offset component and cannot follow a change in level of an amplitude-varying offset component. Therefore, there may be cases where signal decision is erroneously made due to the amplitude-varying offset component.
Referring to
FIGS. 3A and 3B
, a typical example of such an amplitude-varying offset component is a tailing current
303
which flows while gradually decreasing in amplitude after a large intensity of burst mode optical signal
301
has been received by a photodiode. In other words, the tailing current is caused by a large amplitude of burst signal. Since the tailing current
303
flows for a time period longer than a guard time
304
provided between burst signals, the signal current of a subsequent burst optical signal
302
is superposed not only on a do component current I
DC
but also on the tailing current component.
As described above, the conventional automatic offset canceler can eliminate the dc offset component. Therefore, the differential preamplifier produces the positive and negative output signals without the do offset component. However, the amplitude-varying offset component
305
such as the tailing current still remains and unevenly increases the amplitude of the positive and negative output signals obtained from the subsequent burst signal
302
as shown in FIG.
3
B. Such an undesired partial increase in amplitude of input signal becomes more pronounced as a smaller amplitude of the subsequent burst signal.
It is an object of the present invention to provide a burst-mode optical receiver and the automatic offset control therefore which can substantially cancel out not only dc offset component but amplitude-varying offset component.
According to an aspect of the present invention, the optical receiver is comprised of a converter, a detector, and a controller. The converter converts the burst-mode signal into a voltage signal. The detector detects an amplitude-varying offset component from the voltage signal using a discharging characteristic of a capacitor which is charged depending on the voltage signal in synchronization with burst timing of the burst-mode signal. The controller controls the converter to adjust the voltage signal depending on the amplitude-varying offset component.
The detector may be a charge controller which charges the capacitor depending on the voltage signal for a predetermined time period in synchronization with burst timing of the burst-mode signal and then discharges the capacitor with a predetermined time constant to produce the amplitude-varying offset component appearing across the capacitor.
The predetermined time constant may be selected to cause a time-varying curve of the amplitude-varying voltage to be approximately coincident with a time-varying curve of a received burst-mode signal after each burst is terminated.
More specifically, the predetermined time constant may be selected so that the time-varying curve of the amplitude-varying voltage is approximately coincident with a time-varying curve of a tailing current flowing a photo detector after each burst is terminated. In this case, the converter may include a photo detector for converting the burst-mode signal into a current signal and a current-to-voltage converter for converting the current signal into the voltage signal.
According to another aspect of the present invention, an optical receiver includes a converter for converting the burst-mode signal into a voltage signal, a first offset detector for detecting a dc offset component of the voltage signal to produce a first offset signal, a second offset detector for detecting a amplitude-varying offset component of the voltage signal to produce a second offset signal, a combiner for combining the first offset signal and the second offset signal to produce a third offset signal, and a controller for controlling the converter to adjust the voltage signal depending on the third offset signal.
Further, the second offset detector includes a charge controller for charging a capacitor depending on the voltage signal for a predetermined time period in synchronization with burst timing of the burst-mode signal and discharging the capacitor with a predetermined time constant to produce an amplitude-varying voltage appearing across the capacitor, and an amplifier for amplifying the amplitude-varying voltage to produce the second offset signal.
As described above, according to the present invention, the charge controller provides the amplitude-varying voltage which is used to automatically cancel out an amplitude-varying offset component. Therefore, even when a large-intensity optical burst signal is received, data decision can be made properly and reliably without deteriorated receiving sensitivity.
REFERENCES:
patent: 4289399 (1981-09-01), Uchida
patent: 5539779 (1996-07-01), Nagahori
patent: 5737111 (1998-04-01), Mori et al.
patent: 6115163 (2000-09-01), Nobuhara
patent: 0 624 009 (1994-11-01), None
patent: 5-218772 (1993-08-01), None
NEC Corporation
Negash Kinfe-Michael
Young & Thompson
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